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Organic carbon and nitrogen contents and their fractions in soils with onion crops in different management systems.

Introduction

Onion (Allium cepa L.) is a species of the Alliaceae family, which is widely used as human food. China, India, and the United States are the largest onion producers. Brazil is the eighth largest producer, with 1.7 million Mg of onions per year (The Daily Records 2018). The largest onion-producing region in Brazil is in the state of Santa Catarina (SC), which has an annual production of ~500 thousand Mg of onions, representing ~30% of the national production (IBGE 2017).

A significant part of onion crop production in SC is conducted in a conventional tillage system (CTS) with soil turning using plough and harrow, scarifier and harrow, or cultivator (EPAGRI 2013). These managements have caused edaphic degradation (Loss et al. 2015,2017; Santos et al. 2017). Therefore, despite its economic and social benefits, the current onion-production system needs to simultaneously meet economic, social, and environmental requirements to be a sustainable production system.

In this context, some studies have been carried out with onion in a no-tillage system (NTS), where soil tuning is restricted to the planting rows, and the use of single or intercropped plant species produces a biomass that is deposited on the soil surface before planting the onion seedlings (EPAGRI 2013; Silva et al. 2014). The use of NTS in onion crops enhances soil properties by improving total organic carbon (TOC) content, chemical attributes related to soil fertility (Oliveira et al. 2016; Santos et al. 2017), and soil aggregation (Loss et al. 2015, 2017).

The use of crop rotation or succession and soil cover crops (single or intercropped) from different botanical families are also beneficial practices used in onion crops (Silva et al. 2014; Loss et al. 2015, 2017). Santos et al. (2017) and Loss et al. (2017) evaluated chemical and physical attributes of soils cultivated with onion in NTS and CTS with single and intercropped soil cover crops and found that the chemical and physical conditions of the soil in NTS were better when compared with CTS; in addition, there were better edaphic conditions when the onion crops were cultivated on plant residues of intercropped plants from different botanical families, such as intercrops of oilseed radish (Raphanus raphanistrum subsp. sativus (L.) Domin) with rye (Secale cereale L.) or oats (Avena strigosa Schreb.)

Therefore, the use of soil cover crops is necessary for an efficient NTS, since they protect the soil from erosion, participate in the cycling of nutrients, add C and N to the soil, and improve soil aggregation (Santos et al. 2011; Lima Filho et al. 2014; Loss et al. 2015; Vezzani et al. 2018). The plant species used in the NTS will determine the dynamics of production and decomposition of plant material and the soil coverage over time (Oliveira et al. 2016).

NTS improves soil quality compared with CTS (Winck et al. 2014; Vezzani et al. 2018) due to the absence of soil turning, presence of soil physical protection (plant residues), particulate organic matter maintenance (Loss et al. 2012; Winck et al. 2014). However, this improvement in soil quality depends on the crop system used, since the number and alternation of plant species in rotation or succession determines the amount, quality, and form of C and N additions to the soil (Santos et al. 2011; Thivierge et al. 2016; Vezzani et al. 2018). The temporal arrangement of plant species and the amount of shoot and root biomass produced by the soil cover crops (Janzen et al. 1998) can affect the soil C and N contents and stocks in granulometric fractions of organic matter (Six et al. 2004; Sisti et al. 2004; Loss et al. 2012; Winck et al. 2014).

According to Loss et al. (2012) and Winck et al. (2014), the quality of soils subjected to NTS is dependent on the crop system and can be evaluated by their particulate organic matter content due to the function of this fraction in the soil and its sensitivity to different soil management systems. Moreover, it is expected that large aggregates accumulate different amounts of these C and N fractions due to greater biological activity, or have total C and N contents dependent on the management system adopted.

Therefore, the use of NTS with different soil cover crops in rotation or succession with onion crops can increase TOC, total nitrogen (TN), and their fractions in the soil when compared with CTS. In this context, the objective of this study was to evaluate the effect of using soil cover crops in succession and rotation with onion crops in different soil management systems on the TOC, TN, and their fractions in the soil aggregates (2.0-8.0 mm) and bulk soil (<2.0 mm).

Material and methods

The experiment was implemented in April 2007 at the Co. of Agricultural Research and Rural Extension of Santa Catarina, in Ituporanga SC, Brazil. The soil of the region was classified as dystrophic Humic Cambisol (EMBRAPA 2013) or Humic Distrudept (Soil Survey Staff 2006), and its physical and chemical attributes in the 0-10 cm layer were as follows: 410 g [kg.sup.-1] of sand, 264 g [kg.sup.-1] of silt, and 326 g [kg.sup.-1] of clay; pH in [H.sub.2]O of 6.1; exchangeable Ca, Mg, and Al of 6.4, 2.7 and 0.0 cmolc [dm.sup.-3] respectively; available P and K of 42 and 208 mg [dm.sup.3] respectively; and 23.08 g [kg.sup.-1] of TOC (EMBRAPA 1997).

According to the Koppen classification, the climate of the region is Cfa, i.e. subtropical mesothermal humid with hot summers, infrequent frosts, and no defined dry season; it has an average annual temperature of 17.6[degrees]C and average annual precipitation of 1400 mm.

The experiment was conducted in a randomised block design with eight treatments and five replications, with 8.7 [m.sup.2] plots. The treatments consisted of soil management systems for onion crops based on NTS with rotations and successions with different soil cover plant species used to produce biomass, and a CTS.

Oat (A. strigosa Schreb cv. EMBRAPA 139), vetch (Vicia villosa cv. comum), and oilseed radish (R. raphanistrum cv. IPR 116) were sown in the area in 2007, when the experiment was implemented; subsequently, eight treatments (T1-T8) were used with soil cover crops and commercial crops (Table 1).

From 2011, the soil of T7 was prepared using conventional tillage system, with plough and harrow, or cultivator, with succession of maize and onion. The treatments used in 2011, 2012, and 2013 were repeated from 2014 (Table 2).

The plant species chosen for the experiment (Tables 1 and 2) were plants frequently used by regional producers that have good adaptation, seed availability in the market, easy handling, and adequate biomass production for NTS. Thus, the commercial and technical aspects were considered; the experiment was implemented with treatments that could be used by the farmers of the region, making it possible to acquire information about edaphic aspects affected by using NTS in onion crops.

Weed, pest, and disease control were carried out using chemical products that are registered in the Brazilian Ministry of Agriculture, Livestock, and Food Supply for onion crops. Approximately 14 days before onion planting, weeds were killed with glyphosate herbicide (360g [L.sup.-1]) at 4L [ha.sup.-1]. Weed control during the onion cycle was carried out with three applications of herbicides, two of ioxynil (250g [L.sup.-1]) at 1 L [ha.sup.-1] at 35 and 65 days after seedling transplantation (DAT) and one of clethodim (240g [L.sup.-1]) at 0.4 L [ha.sup.-1] at 85 DAT. The control of pests, especially Thrips tabaci Lind., was carried out with three applications of insecticides, one of imidacloprid (700g [L.sup.-1]) at 0.1kg [ha.sup.-1] at 30 days after planting and two of lambda-cyhalothrin (50g[L.sup.-1]) at 0.1 L [ha.sup.-1] approximately at 60 and 81 DAT. The control of fungal diseases, mainly mildew (Peronospora destructor) and Alternaria solani was carried out with six applications of fungicides, four of metalaxyl (40g [L.sup.-1]) + mancozeb (640g [L.sup.-1]) at 35, 50, 65 and 80 DAT and two of tebuconazole (200 mL [L.sup.-1]) + trifloxystrobin (100 mL [L.sup.-1]) at 80 and 94 DAT. All applications were performed using personal protective equipment.

The soil of the experiment area had been cultivated using a conservationist production system since 1995, when the last liming was carried out to raise the pH to 6.0 using dolomitic limestone. It was incorporated by ploughing and harrowing the soil to a depth of 20 cm. Since then, it was managed in NTS, with soil preparation restricted to the planting rows, except T7, which was managed in CTS (Table 2).

Fertilisation was carried out only for onion and maize crops based on soil analyses during the experiment and according to recommendations for these crops (CQFSRS/SC, 2004). Fertilisation for onion crops consisted of 75 kg [ha.sup.-1] of N, 120 kg [ha.sup.-1] of P205, 60 kg [ha.sup.-1] of [K.sub.2]O, using the 05-20-10 NPK formulation, or triple superphosphate, potassium chloride, and ammonium nitrate. The P rates were very high in 2010 (CQFSRS/SC, 2004); thus, only 50 kg [ha.sup.-1] of P was used for the first, and 80 kg [ha.sup.-1] for the following onion crops. P and K were applied to the onion crops at planting, and N was applied at planting (15 Kg N [ha.sup.-1]) and at 45, 65, and 85 days after transplantation (topdressing) of the onion seedlings, using ammonium nitrate. Thirty kilograms per hectare of sulfur (calcium sulfate) were applied in 2014 at 45 days after the transplant of the onion seedlings. P and K were not applied to maize crops because the soil presented high contents of these nutrients; nitrogen was applied using 90 kg [ha.sup.-1] of urea when the maize reached six to eight leaves.

The soil cover crops were killed, and furrows were opened with a machine adapted for onion planting in NTS; the seedlings (cultivar Bola Precoce Empasc-352) were manually transplanted with 0.40 m between rows and 0.10 m between plants. There were seven rows of onion per plot and 30 plants per row.

The shoot dry weight (SDW) of the commercial plants and soil cover plants, and the onion bulb yield of the eight treatments were evaluated in 2014 (Table 3). The SDW was evaluated in July 2014; therefore, it was the plant residues of commercial crops and soil cover crops implemented in the summer of 2013 and winter of 2014 (Table 3). Table 3 shows the average onion bulb yield of 2008 to 2014. T1, T4, T7, and T8 had winter fallow (Table 2); consequently, the shoot dry biomass added was from weeds, crop residues, and soil cover crops from the summer of 2013. The main weed families found were Amaranthaceae, Apiaceae, Asteraceae, Caryophyllaceae, Convolvulaceae, Cyperaceae, Euphorbiaceae, Lamiaceae, Malvaceae, Oxalidaceae, Plantaginaceae, Poaceae, Polygonaceae, and Rubiaceae.

Seven years after the implementation of the experiment in July 2014, undisturbed soil samples were collected; a hole of 40 x 40 x 40 cm was opened between the rows of each onion plot using a spade for collecting the soil samples in the 0.0-5.0, 5.0-10.0, and 10.0-20.0 cm layers. The samples were then placed in plastic bags and sent to the laboratory for analyses.

The soil samples were air-dried and manually disaggregated by following slits or weak points and passed through 8.00-, 4.00-, and 2.00-mm mesh sieves, according to the methodology adapted by EMBRAPA (1997). The soil aggregates that passed through the 8.00-mm mesh sieve and were retained in the 4.00-mm mesh sieve were used to evaluate the soil aggregates. The bulk soil that passed through the 4.00-mm mesh sieve was air-dried, disaggregated, and sieved in a 2.00-mm mesh sieve to obtain the air-dried fine earth (ADFE) from the soil (bulk soil with [empty set] <2.0 mm).

Soil aggregates that were retained in the 4.00-mm mesh sieve were air-dried, disaggregated, and sieved in a 2.00-mm mesh sieve to obtain the ADFE from the aggregates (8.00mm > [empty set] [greater than or equal to] 2.0mm), which was used to determine the C, N, and granulometric fractions of organic matter in the aggregates. TOC and TN contents in the ADFE from the bulk soil ([empty set] <2.0 mm) and ADFE from the soil aggregates (8.00 mm > [empty set] [greater than or equal to] 2.0 mm) were determined using a dry combustion elemental analyser CHN (FlashEA 1112, Thermo Finnigan).

The granulometric fractionation of the soil aggregates and bulk soil was performed according to Cambardella and Elliott (1992) to obtain the particulate organic carbon (O[C.sub.P]) and particulate organic nitrogen (O[N.sub.P]), which are fractions with sizes smaller than 0.053 mm. The contents of mineral-associated organic carbon (O[C.sub.M]) and mineral-associated organic nitrogen (O[N.sub.M]) were determined by the difference between the TOC-to-TN ratio and the O[C.sub.p]-to-O[N.sub.p] ratio.

The results were analysed for normality and homogeneity of the data by the Lilliefors (Lilliefors 1967) and Bartlett (Bartlett 1937) tests respectively. The data were subjected to analysis of variance (F test) and, when the effects were significant, the means were compared by the Scott-Knott test at 5% probability using the Sisvar program. Statistical analyses were performed for the eight treatments, soil aggregates, and bulk soil. Subsequently, the data of each treatment were subjected to statistical analysis independently, comparing the results of the soil aggregates and bulk soil by the Student's t-test (1.s.d.) at 5%.

Results

TOC and TN contents in the bulk soil and soil aggregates

The TOC contents were 20.6-43.4 g [kg.sup.-1] in the bulk soil and 21.3-39.6 g [kg.sup.-1] in the soil aggregates. The TN contents were 1.7-4.6 g [kg.sup.-1] in the bulk soil and 1.5-3.3 g [kg.sup.-1] in the soil aggregates. T7 presented the lowest TOC in bulk soil (0.0-20.0 cm) and aggregates (0.0-5.0 cm), as well as the lowest TN in bulk soil (0.0-10.0 cm) and aggregates (0.0-5.0 cm). T6 had the highest TOC (0.0-5.0 cm) and TN (0.0-10.0 cm) in bulk soil (Table 4).

The aggregates had the highest TOC and TN contents in T2, T3, T5, and T6 treatments. The soil TOC content was higher in T3 than in T2, T5, and T6. TOC in aggregates (5.0-10.0 and 10.0-20.0 cm) and TN in bulk soil (10.0-20.0 cm) and aggregates (5.0-10.0 and 10.0-20.0 cm) in the NTS and CTS treatments were similar (Table 4).

TOC in bulk soil and aggregates presented low differences between treatments, with few variations in the 5.0-10.0 and 10.0-20.0 cm layers. However, T3, T4, T5, and T7 stood out in the 0.0-5.0cm layer. T6 and T7 stood out in the 5.0-10.0cm layer with higher TOC in aggregates, compared with that found in bulk soil. T6 and T8 had higher TOC contents in bulk soil (0.0-5.0 cm) compared with aggregates. The highest TN contents were found, in general, in bulk soil when compared with aggregates (Table 4).

Granulometric fractionation of the bulk soil and soil aggregates

The O[C.sub.P] contents were 4.67-13.55 g [kg.sup.-1] in bulk soil and 2.51-12.72 g [kg.sup.-1] in aggregates. The O[C.sub.M] contents were 14.42-29.94 g [kg.sup.-1] in bulk soil and 15.80-26.96 g [kg.sup.-1] in aggregates. The lowest O[C.sub.P] content in bulk soil and aggregates (0.0-5.0 cm) and the lowest O[C.sub.M] content in bulk soil (0.0-10.0 cm) were found in T7. The lowest O[C.sub.M] contents in aggregates were found in the 0.0-5.0 cm layer in T4 and T8. The highest O[C.sub.P] (0.5-10.0 cm) and O[C.sub.M] (0.0-5.0 cm) in aggregates were found in T3. The highest O[C.sub.P] contents in bulk soil (0.0-5.0 cm) were found in T3 and T6. The highest O[C.sub.M] in bulk soil (0.0-5.0 cm) was found in T6 (Table 5).

The O[N.sub.P] contents were 0.04-0.78 g [kg.sup.-1] in bulk soil, and 0.05-0.48 g [kg.sup.-1] in aggregates. The O[N.sub.M] contents were 1.60-3.88 g [kg.sup.-1] in bulk soil and 1.48-2.96 g [kg.sup.-1] in aggregates. The lowest O[N.sub.P] contents in bulk soil (0.0-10.0 cm) and aggregates in 0.0-5.0 cm and lowest O[N.sub.M] in bulk soil (0.0-20.0 cm) were found in T7. However, this treatment had the highest O[N.sub.P] contents in aggregates in the 5.0-10.0 cm layer and in bulk soil in the 10.0-20.0 cm layer. The highest O[N.sub.P] contents were found in bulk soil in the 0.0-5.0 cm layer and in aggregates in the 10.0-20.0 cm layer; and the highest O[N.sub.M] contents in bulk soil (0.0-5.0 cm) were found in T6 (Table 6).

Discussion

TOC and TN contents in the bulk soil and soil aggregates

The lowest TOC and TN contents found in bulk soil and aggregates in T7 were due to the soil management and the lower plant diversity used in this treatment. T7 received biomass from the maize residues and weeds in the fallow (Table 3). However, the soil preparation practices caused fragmentation of these residues and accelerated their decomposition. They also caused rupture of aggregates, exposing the organic C and N that were previously protected and favouring their decomposition by soil microbiota. CTS increase the organic matter mineralisation rate, which decreased the TOC and TN contents (Boddey et al. 2010; Busari et al. 2015). The lowest onion bulb yields were found in T7 (Table 3), which confirms the lower TOC and TN contents found this treatment (Table 4).

Loss et al. (2015) evaluated chemical and physical attributes in soil aggregates and found similar results. They evaluated the effects of using soil cover crops (single and intercropped) for onion crops in NTS for 5 years on the soil aggregation and TOC in the soil aggregates, compared with the use of onion crops in CTS for 37 years. They found that the use of soil cover crops with onion in NTS increased soil aggregation, the amount of macroaggregates, and the soil TOC content in the aggregates (0.0-5.0 cm soil depth).

Considering the soil TOC content at the beginning of the experiment in 2007 (23.08 g [kg.sup.-1] in the 0.0-10.0 cm layer), the TOC in bulk soil (0.0-10.0 cm) increased in all treatments, except in T7 (5.0-10.0 cm). Treatments in NTS had more pronounced increases than that in CTS. Studies have reported reductions of soil TOC and TN contents in CTS when compared with NTS, reduced soil tillage, pasture, natural vegetation, and native forest (Tivet et al. 2013; Silva et al. 2014; Loss et al. 2015; Santos et al. 2017). Treatments in NTS have constant deposition of plant residues on the soil surface, favouring the maintenance and increasing of TOC contents and, consequently, increasing the TN contents (Silva et al. 2014; Vezzani et al. 2018).

The highest TOC (0.0-5.0 cm) and TN (0.0-10.0 cm) contents found in the bulk soil of T6 may be due to the combination of plant species from different botanical families--velvet bean (legume) and rye (grass). The use of velvet bean may explain the high TN, since this species presents a low C: N ratio (average of 16.5), average dry matter of 7.5 Mg [ha.sup.-1], and great N biological fixation (120 to 210kg [ha.sup.-1] [year.sup.-1] of N) and nutrient cycling (Lima Filho et al. 2014). Rye has a fasciculate and dense root system that develops to depths of up to 122 cm, dry matter with high C : N ratio (average of 30.5), and average dry matter of 4.5 Mg [ha.sup.-1] (Weaver 1926; Lima Filho et al. 2014).

The use of soil cover crops, especially grasses, protect the soil against climatic events and favours carbon input, mainly by rhizodeposition (Thivierge et al. 2016). Single crops of legumes, and especially intercrops of legumes, can absorb nutrients from deep soil layers (1.0 and 1.5 m) (Gathumbi et al. 2003). Rye crops can accumulate 91-100 kg N [ha.sup.-1] (Fageria et al. 2005; Oliveira et al. 2016), but with slow N mineralisation. Thus, the high amount of dry matter produced by the velvet bean combined with the high C: N ratio of the rye dry matter can explain the higher TOC and TN contents in bulk soil in T6. The higher onion bulb yield in T6 in 2014 (Table 3) confirms its higher TOC and TN contents (Table 4).

According to Giacomini et al. (2003), intercrops with different plant species produce biomass with an intermediate C : N ratio to that of single crops. Doneda et al. (2012) evaluated rye intercropped with oilseed radish, and oat intercropped with oilseed radish and found a slower decomposition rate of plant residues when compared with single crops and an intermediate C: N ratio of the biomass. Thus, the decomposition rate of plant residues can be altered to simultaneously provide more efficient and lasting soil coverage and better synchronisation between supply and demand of nutrients of the crops in succession, especially N (ASHS, 2010). Boddey et al. (2010) evaluated TOC stocks in three long-term soybean experiments using NTS and CTS with crop rotation in Latosols in southern Brazil and found significant increases in TOC stocks in NTS when compared with CTS, and in areas with legumes in the crop rotation. These results confirm those found in the present study, especially considering the bulk soil of T6, which denotes the importance of N as a component of the soil organic matter (SOM) humification process and soil carbon retention (Christopher and Lai 2007).

The highest TN and TOC contents in soil aggregates of T3, T2, T5, and T6 can be attributed to the use of species of the family Poaceae (grasses) in T3 and T5 and the combination of grass and legume species in T2 and T6. T2 is a crop rotation--vetch, maize, rye intercropped with oilseed radish, and common bean. Oats stood out because of its fasciculate root system that generally reaches a depth of 76 cm, producing on average 6 Mg [ha.sup.-1] of dry matter with a high C : N ratio (average of 31.5). Rye has a high nutrient cycling capacity, fasciculate root system that develops to a depth of 122 cm, and an average dry matter yield of 4.5 Mg [ha.sup.-1] with a high C : N ratio (average of 30.5). Maize has a very extensive and branched root system, reaching depths of 1.8 m, producing ~6 Mg [ha.sup.-1] of dry matter with a high C: N ratio (average of 52) (Weaver 1926; Lima Filho et al. 2014). Therefore, the combination of deep and dense root systems that perform C deposition and large dry matter yields with high C : N ratio can explain the higher TOC contents found in T3. T5 had oat, rye, and millet, which also has a profuse and deep root system, reaching 200 cm (Norman et al. 1995). T2 and T6 had combinations of plant species, grasses, and legumes, and T2 had a crop rotation. These combinations favour the balance of C: N ratio of the plant biomass with a consequent increase of TOC and TN contents.

The input of C and N in the soil occurs not only via decomposition of the shoot of the soil cover crops, but also via rhizodeposition. Thivierge et al. (2016) evaluated the contribution of the root systems of maize, sorghum, and millet to the soil C contents, and found that the inputs of C from maize crop residues after harvest (243 g C [m.sup.-2]) was higher than those of the sorghum (197 g C [m.sup.-2]) and millet (131 g C [m.sup.-2]), and large part of this C was derived from fine roots with a diameter of less than 0.5 mm.

Amado et al. (2001) evaluated the potential of soil cover crops and plants to accumulate C and N in NTS and found that the use of legumes, combined with a greater diversity of species in succession or rotation, significantly increased the C and N retention in the soil. This confirms the highest TOC and TN contents in treatments with combination of plant species from different families. Jantalia et al. (2003) evaluated crop systems with rotation and succession including legume and grass species used as green manuring and mulching. They found higher C and N stocks in NTS with crop rotation systems using plant species from different families compared with CTS with succession of wheat and soybean; there were no TOC and TN stock increases due to soil cover plants in the CTS.

The absence of differences in TOC in aggregates (5.0-10.0 and 10.0-20.0 cm) and TN in bulk soil (10.0-20.0 cm) and aggregates (5.0-10.0 and 10.0-20.0 cm) between treatments in NTS and CTS was due to soil turning in the CTS, which inverts the soil layers. Thus, plant residues in the soil surface layer, which present high SOM contents, are incorporated into deeper soil layers in CTS (Loss et al. 2015), changing the nutrient contents of the soil profile and equating them to those found in NTS (Loss et al. 2015; Santos et al. 2017). The absence of differences in TOC and TN in deeper layers of the treatments in NTS is probably due to the higher contribution of soil cover plants and crops to the soil surface layer and the absence of soil turning. Sisti et al. (2004) conducted a long-term experiment (15 years) with crop rotation and succession with legumes and grasses in NTS and found differences in TOC and TN contents only in the 0.0-5.0 cm layer; they reported that the soil C and N stocks are dependent on the soil tillage system and soil cover crop species used.

The physical and chemical protection of the SOM by the soil aggregates explains the higher TOC contents in soil aggregates, compared with bulk soil, in T3, T4, T5, and T7 treatments in the 0.0-5.0 cm layer, and of T6 and T7 in the 5.0-10.0 cm layer. The physical protection of the SOM by occlusion due to the soil aggregates makes it difficult for microorganisms and their enzymes to interact with the organic substrate; it is a physical barrier that reduces the 02 availability for the oxidative processes of decomposition (Baldock et al. 1992; Balesdent et al. 2000).

Zhong et al. (2017) found similar results and reported that soil aggregates present less accessibility to microorganisms to organic substrates, decreasing the soil microbial activity and physically protecting the C and N from decomposition. These authors conducted an experiment on a Red Latosol in forest plantation (Schima sp.) in south-western China, evaluating the physical protection of organic C by soil aggregates and reductions in TOC loss considering TOC, TN, and C and N of microbial biomass, dissolved organic C, and hot water extractable organic C in aggregates and bulk soil. They found 61.79% to 69.86% less N of microbial biomass in aggregates than in bulk soil; 20.69%, 15.74%, and 13.36% less C of microbial biomass in aggregates of 1-2, 2-5, and 5-8 mm respectively; and41.02%-66.40%, and 91.30%-104.45% higher concentrations of dissolved organic C and hot water extractable organic C in aggregates than in bulk soil respectively. These results denote the decreased microbial activity due to the physical protection of the organic matter by the soil aggregates, which prevents organic C decomposition and results in higher concentrations of dissolved organic C and hot water extractable organic C, compared with bulk soil.

Granulometric fractionation of the bulk soil and soil aggregates

The lowest O[C.sub.P] contents in bulk soil and aggregates (0.0-5.0 cm), and the lowest O[C.sub.M] contents in bulk soil (0.0-10.0 cm) in T7 were due to the soil management adopted in this treatment, which was conducted using plough and harrow, or cultivator, which ruptured and fragmented the soil aggregates. This management exposed the SOM that was protected within the aggregates to microbial decomposition (Meurer 2012; Loss et al. 2014), decreasing the O[C.sub.P], which confirms that the CTS changes the soil aggregation and increases the SOM decomposition rate, causing a decrease in O[C.sub.P] and O[C.sub.M] contents when compared with NTS. These results explain the lower TOC contents in bulk soil (0.0-10.0 cm) and aggregates (0.0-5.0 cm) in this treatment.

The higher O[C.sub.P] (5.0-10.0 cm) and O[C.sub.M] (0.0-5.0 cm) contents in aggregates in T3 is explained by the soil cover crops (rye, oat, and maize) used, which resulted in high amount of shoot dry matter with high C : N ratio, and the decomposition of organic compounds (Thivierge et al. 2016; Lima Filho et al. 2014). These results confirm the higher TOC content in aggregates (0.0-5.0 cm) in this treatment.

The highest O[C.sub.P] and O[C.sub.M] contents in bulk soil (0.0-5.0 cm) in T6 is explained by the high amount of dry matter produced by the velvet bean combined with the high C: N ratio of the rye dry matter (Weaver 1926; Lima Filho et al. 2014). These results confirm the higher TOC contents in bulk soil (0.0-5.0 cm) in this treatment.

Compared with T7, the T1 treatment (succession of onion and maize in NTS) presented higher TOC contents in bulk soil (0.0-20.0 cm) and aggregates (0.0-5.0 cm), TN in bulk soil (0.0-10.0 cm) and in aggregates (0.0-5.0 cm), O[C.sub.P] in bulk soil (0.0-5.0 cm) and aggregates (0.0-10.0 cm), O[C.sub.M] in bulk soil (0.0-10.0 cm), O[N.sub.P] in bulk soil (0.0-10.0 cm) and aggregates (0.0-5.0 cm), and O[N.sub.M] in bulk soil (0.0-20.0 cm). Since the same succession (onion and maize) was used in both systems, these results denote the importance of conservationist management practices, such as NTS, to improve soil chemical attributes (TOC and TN) and granulometric fractions of SOM when compared with CTS. Moreover, T1 had higher onion yield than T7 (Table 3).

The results obtained in T6, compared with the results of T3 and T5 (both with grass species only) and T4 (with legume species only) showed higher TOC in bulk soil (0.0-5.0 cm), TN in bulk soil (0.0-10.0 cm), O[C.sub.P] in bulk soil (0.0-5.0 cm, except T3), O[C.sub.P] in aggregates (10.0-20.0 cm, except T3), O[C.sub.m] in bulk soil (0.0-5.0 cm), O[N.sub.P] in bulk soil and in aggregates (0.0-5.0 and 10.0-20.0 cm respectively), and O[N.sub.M] in bulk soil (0.0-10.0 cm). These results indicate the importance of using plant species of different botanical families, such as grasses and legumes, for the improvement of soil chemical attributes and granulometric fractions of the SOM.

T8 presented, in general, similar or greater TOC and TN contents than T1, T2, T4, and T5 in the 5.0-10.0 and 10.0-20.0 cm layers in aggregates and bulk soil. However, the O[C.sub.P] contents in these layers differed between treatments only in aggregates, with higher O[C.sub.P] contents in T8 compared with T1, T2, T3, and T4. The O[N.sub.P] contents in bulk soil and aggregates of T1, T2, T3, T4, and T8 were different. However, T8 presented higher O[N.sub.P] contents in the aggregates when compared to T1, T2, T4, and T5 (5.0-20.0cm). Therefore, using plant species of different botanical families (pearl millet, velvet bean, and sunflower) is important to increase the soil particulate C and N fractions. The greater variation of O[C.sub.P] and O[N.sub.P] in the treatments in aggregates, compared with bulk soil, confirms the importance of particulate organic matter for nucleation and formation of soil aggregates, especially macroaggregates (Golchin et al. 1994; Six et al. 2004).

Winck et al. (2014) evaluated soil TOC and TN stocks and granulometric fractions of the SOM in six soil management systems with different crop rotations in NTS. They found that soil TOC and TN stocks, and granulometric fractions of the SOM increased with an increase in the number of species in the rotation that have longer cycles and high organic matter input to the soil, consequently favouring the soil functions by increasing soil quality.

Regarding the C and N contents in bulk soil and aggregates of each treatment, the O[C.sub.M] was, in general, higher in aggregates; and O[C.sub.P], O[N.sub.P] and O[N.sub.M] were, in general, higher in bulk soil. T8 had higher O[C.sub.P] (0.0-20.0 cm) and O[N.sub.P] (5.0-10.0 cm) in aggregates than in bulk soil. The highest O[C.sub.M] contents in aggregates were due to the physical and chemical protection of the SOM by the aggregates. The highest O[C.sub.P] and O[N.sub.P] contents in aggregates in T8 may be due to the higher biomass produced in this treatment compared with the others (Table 3).

Conclusions

Succession of maize and onion in CTS reduced TOC, O[C.sub.P], O[C.sub.M], TN, O[N.sub.P], and O[N.sub.M] in soil aggregates and bulk soil when compared with NTS for onion crops in the topsoil. Velvet bean and rye in these successions increased the TOC, TN, O[C.sub.P], O[C.sub.M], O[N.sub.P] and O[N.sub.M] contents in aggregates and bulk soil when compared with the successions with only grasses or only legumes. Succession of maize and onion crops in NTS increased TOC and TN contents, and the granulometric fractions of the organic matter when compared with CTS with the same succession. Succession of intercrop of pearl millet, velvet bean, and sunflower (summer) in NTS increased the O[C.sub.P] and O[N.sub.P] contents in the soil aggregates when compared with succession of maize and onion in CTS, rotation of crops with winter soil cover crops in NTS, succession of summer legumes in NTS, and rotation of summer grass, winter grasses, in NTS.

In general, TOC and O[C.sub.M] contents were higher in soil aggregates, whereas TN, O[C.sub.P], O[N.sub.P], and O[N.sub.M] contents are higher in bulk soil. The main changes due to management systems and combinations of soil cover crops were observed in the particulate fractions, mainly in the soil aggregates.

Conflicts of interest

The authors declare no conflicts of interest.

https://doi.org/10.1071/SR18167

Acknowledgements

The authors thank the National Council for Scientific and Technological Development--CNPq (Process n[degrees] 302603/2015-8 and 403949/2016-5); the Fundacao Agrisus (Process no. PA 1622/15); and the Experimental Station (EPAGRI) of Ituporanga, Santa Catarina, Brazil, for the availability of the experimental area. This study was funded in part by the Coordination of Improvement of Higher Education Personnel--Brazil (CAPES)--Finance Code 001.

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Handling Editor: Chengrong Chen

Lucas Borges Ferreira (A), Arcangelo Loss (iD) (A,E), Lucas Dupont Giumbelli (A), Barbara Santos Ventura (A), Monique Souza (A), Alvaro Luiz Mafra (B), Claudinei Kurtz (C), Jucinei Jose Comin (A), and Gustavo Brunetto (D)

(A) Federal University of Santa Catarina, Florianopolis SC, Brazil.

(B) State University of Santa Catarina, Lages SC, Brazil.

(C) Research and Agricultural Extension Company of Santa Catarina, Ituporanga SC, Brazil.

(D) Federal University of Santa Maria, Santa Maria RS, Brazil.

(E) Corresponding author. Email: arcangelo.loss@ufsc.br
Table 1. Species used in rotation or succession with onion crops in
different soil tillage systems from 2007 to 2010. Ituporanga SC, Brazil

Species are as follows: oat (Avena slrigosa Schreb), onion
(Allium cepa L.), rye (Secale cereale L.), showy rattlebox
(Crotalaria spectabilis Roth), vetch (Vicia villosa Roth), common
bean (Phaseolus vulgaris L.), jack bean (Canavalia ensiformis (L.)
DC.), sunflower (Helianthus annuus L.), maize (Zea mays L.), pearl
millet (Pennisetum glaucum (L.) R.Br.), velvet bean (Mucunapruriens
var. utilis (Wall, ex Wight) L.H. Bailey), oilseed radish (Raphanus
raphanislrum subsp. sativus (L.) Domin), and barley (Hordeum
vulgare L.). T1, succession of onion, and maize in no-tillage
system (NTS); T2, rotation of soil cover crops (winter), and
biennial onion in NTS; T3, rotation of maize, winter grasses, and
onion in NTS; T4, succession of summer legume and annual onion in
NTS; T5, rotation of summer grass, winter grasses, and annual onion
in NTS; T6, succession of summer legume, winter grass, and annual
onion in NTS; T7, succession of maize and onion in conventional
tillage system (CTS); T8, succession of intercrops of soil cover
crops (summer), and annual onion in NTS

T                       2007

          Winter               Summer

T1    Oat + Vetch +            Maize
      Oilseed radish

T2    Oat + Vetch +            Maize
      Oilseed radish

T3    Oat + Vetch +            Maize
      Oilseed radish

T4    Oat + Vetch +            Maize
      Oilseed radish

T5    Oat + Vetch +    Onion   Pearl millet
      Oilseed radish

T6    Oat + Vetch +    Onion   Jack bean
      Oilseed radish

T7    Oat + Vetch +    Onion   Jack bean +
      Oilseed radish           Pearl millet

T8    Oat + Vetch +    Onion   Sunflower
      Oilseed radish

T                      2008

              Winter           Summer

T1    Fallow           Onion   Maize

T2    Oat + Oilseed    Onion   Sunflower
      radish + Rye

T3    Oat + Oilseed    Onion   Maize
      radish

T4    Oat + Oilseed    Onion   Velvet
      radish + Rye             bean

T5    Oilseed radish   Onion   Pearl millet

T6    Rye              Onion   Velvet bean

T7    Oat              Onion   Showy rattlebox

T8    Oat + Rye        Onion   Sunflower +
                               Velvet bean +
                               Pearl millet

T                 2009

          Winter       Summer

T1    Fallow Onion     Maize

T2    Oat + Vetch +    Common
      Oilseed radish   bean

T3    Vetch            Maize

T4    Rye              Maize

T5    Oat + Vetch +    Maize
      Oilseed radish

T6    Onion            Velvet
                       bean

T7    Rye              Maize

T8    Vetch            Maize

T                   2010

             Winter            Summer

T1    Fallow           Onion   Maize

T2    Rye + Oilseed    Onion   Maize
      radish

T3    Rye              Onion   Maize

T4    Oilseed radish   Onion   Velvet bean

T5    Barley           Onion   Pearl millet

T6    Rye              Onion   Velvet bean

T7    Oat              Onion   Showy rattlebox

T8    Rye + Oat +      Onion   Pearl millet +
      Oilseed                  Velvet bean +
      radish                   Sunflower

Table 2. Species used in rotation or succession with onion crops
from 2011 to 2013. Ituporanga SC, Brazil

Species are as follows:
oat (Avena strigosa Schreb), onion (Allium cepa L.), rye (Secale
cereale L.), Vetch (Vicia villosa Roth), common bean (Phaseolus
vulgaris L.), sunflower (Helianthus annuus L.), Maize (Zea mays
L.), pearl millet (Pennisetum glaucum (L.) R.Br.), velvet bean
(Mucuna pruriens var. utilis (Wall, ex Wight) L.H. Bailey) and
oilseed radish (Raphanus raphanistrum subsp. sativus (L.) Domin).
T, treatment; T1, succession of onion, and maize in no-tillage
system (NTS); T2, rotation of soil cover crops (winter), and
biennial onion in NTS; T3, rotation of maize, winter grasses, and
onion in NTS; T4, succession of summer legume and annual onion in
NTS; T5, rotation of summer grass, winter grasses, and annual onion
in NTS; T6, succession of summer legume, winter grass, and annual
onion in NTS; T7, succession of maize and onion in conventional
tillage system (CTS); T8, succession of intercrops of soil cover
crops (summer), and annual onion in NTS

T                2011

          Winter       Summer

T1    Fallow   Onion   Maize

T2         Vetch       Maize

T3    Rye      Onion   Maize

T4    Fallow   Onion   Velvet bean

T5    Rye      Onion   Pearl millet

T6    Rye      Onion   Velvet bean

T7    Fallow   Onion   Maize

T8    Fallow   Onion   Pearl millet +
                       Velvet bean +
                       Sunflower

T                   2012

             Winter           Summer

T1    Fallow          Onion   Maize

T2    Rye + Oilseed   Onion   Maize
      radish

T3    Oat             Onion   Maize

T4    Fallow          Onion   Velvet bean

T5    Oat             Onion   Pearl millet

T6    Rye             Onion   Velvet bean

T7    Fallow          Onion   Maize

T8    Fallow          Onion   Pearl millet +
                              Velvet bean +
                              Sunflower

T                    2013

               Winter        Summer

T1    Fallow         Onion   Maize

T2    Oilseed                Common bean
            radish + Rye

T3    Rye            Onion   Maize

T4    Fallow         Onion   Velvet bean

T5    Rye            Onion   Pearl millet

T6    Rye            Onion   Velvet bean

T7    Fallow         Onion   Maize

T8    Fallow         Onion   Pearl millet +
                             Velvet bean +
                             Sunflower

Table 3. Shoot dry weight of soil cover crops, onion bulb yield in
2014, and annual average of onion bulb yield in 2008 to 2014

T1, succession of onion, and maize in no-tillage system (NTS); T2,
rotation of soil cover crops (winter), and biennial onion in NTS;
T3, rotation of maize, winter grasses, and onion in NTS; T4,
succession of summer legume and annual onion in NTS; T5, rotation
of summer grass, winter grasses, and annual onion in NTS; T6,
succession of summer legume, winter grass, and annual onion in NTS;
T7, succession of maize and onion in conventional tillage system
(CTS); T8, succession of intercrops of soil cover crops (summer),
and annual onion in NTS; *, virtually no common bean plant residues
were found in the evaluation period; **, there was no onion
production in 2014 (biannual onion crops)

            T1     T2     T3     T4     T5     T6     T7     T8

                    Shoot dry weight (kg [ha.sup.-1])

           1652    *     4000   5196   3684   5508   1516   8328

                    Onion bulb yield (Mg [ha.sup.-1])

2014       27.2    **    28.6   23.1   29.5   30.8   18.8   26.2

2008 to    30.6   34.9   34.7   34.1   34.8   33.5   28.1   34.4
2014

Table 4. Total organic carbon (TOC) and total nitrogen (TN) in soil
bulk and soil aggregates of a Humic Cambisol subjected to
no-tillage and conventional tillage systems with onion crops using
crop rotations and successions. Ituporanga SC, Brazil

Means followed by the same uppercase letter in the column did not
differ significantly between treatments for soil bulk and
aggregates by the Scott-Knott test at 5%. Means followed by the
same lowercase letter in the row did not differ significantly
between soil bulk and aggregates for each treatment by the
Student's t-test (l.s.d.) at 5%. CV, coefficient of variation; T1,
succession of onion, and maize in no-tillage system (NTS); T2,
rotation of soil cover crops (winter), and biennial onion in NTS;
T3, rotation of maize, winter grasses, and onion in NTS; T4,
succession of summer legume and annual onion in NTS; T5, rotation
of summer grass, winter grasses, and annual onion in NTS; T6,
succession of summer legume, winter grass, and annual onion in NTS;
T7, succession of maize and onion in conventional tillage system
(CTS); T8, succession of intercrops of soil cover crops (summer)

             TOC (g [kg.sup.-1])

       Soil bulk   Aggregates    CV%

              0.0-5.0 cm layer

T1      32.92Ba     31.48Ca     4.31
T2      34.37Ba     34.30Ba     7.71
T3      34.13Bb     39.64Aa     13.25
T4      29.63Cb     31.91Ca     4.56
T5      30.96Cb     34.85Ba     2.78
T6      43.49Aa     33.88Bb     5.14
T7      24.96Db     26.46Da     4.54
T8      33.86Ba     30.47Cb     3.86
CV%      6.71         7.10

              5.0-10.0cm layer

T1      25.69Aa     25.40Aa     7.39
T2      27.50Aa     26.26Aa     6.07
T3      26.09Aa     25.69Aa     7.51
T4      28.10Aa     26.43Aa     4.52
T5      27.47Aa     27.70Aa     5.54
T6      25.15Ab     26.74Aa     2.96
T7      20.64Bb     25.02Aa     3.39
T8      26.58Aa     25.16Aa     4.16
CV%      5.48         5.53

            10.0-20.0 cm layer

T1      22.08Ba     21.93Aa     5.78
T2      23.56Aa     23.16Aa     6.89
T3      23.39Aa     22.91Aa     4.23
T4      22.17Ba     23.76Aa     5.61
T5      23.73Aa     23.62Aa     3.89
T6      24.39Aa     23.18Aa     4.65
T7      20.67Ca     23.42Aa     6.51
T8      22.83Ba     21.32Aa     5.28
CV%      5.28         5.54

             TN (g [kg.sup.-1])

       Soil bulk   Aggregates    CV%

             0.0-5.0 cm layer

T1      3.27Ca       2.70Bb     2.05
T2      3.72Ba       3.12Ab     4.99
T3      3.01Ca       3.27Aa     8.55
T4      3.52Ca       2.76Bb     6.43
T5      3.22Ca       3.11Aa     5.72
T6      4.66Aa       3.30Ab     10.82
T7      2.37Da       2.26Ca     3.26
T8      3.48Ca       2.95Ab     7.70
CV%      10.32        7.38

              5.0-10.0cm layer

T1      2.34Ba       2.07Ab     5.75
T2      2.40Ba       2.08Ab     5.35
T3      2.41Ba       2.23Ab     3.11
T4      2.46Ba       2.15Ab     4.57
T5      2.49Ba       2.23Ab     3.16
T6      2.59Aa       2.17Ab     3.40
T7      1.81Cb       2.22Aa     6.08
T8      2.52Ba       2.09Ab     7.91
CV%      7.34         5.04

             10.0-20.0 cm layer

T1      1.81Aa       1.62Ab     4.13
T2      1.85Aa       1.64Ab     6.17
T3      1.86Aa       1.66Ab     5.81
T4      1.77Aa       1.64Aa     9.08
T5      1.94Aa       1.75Aa     6.97
T6      1.92Aa       1.67Ab     3.78
T7      1.73Aa       1.75Aa     8.87
T8      1.88Aa       1.58Ab     5.05
CV%      2.03         8.02

Table 5. Particulate organic carbon ([OC.sub.P]) and mineral-associated
organic carbon ([OC.sub.M]) in soil bulk and soil aggregates of a Humic
Cambisol subjected to no-tillage and conventional tillage systems
with onion crops using crop rotations and successions. Ituporanga
SC, Brazil

Means followed by the same uppercase letter in the column did not
differ significantly between treatments for soil bulk and
aggregates by the Scott-Knott test at 5%. Means followed by the
same lowercase letter in the row did not differ significantly
between soil bulk and aggregates for each treatment by the
Student's t-test (l.s.d.) at 5%. CV, coefficient of variation; T1,
succession of onion, and maize in no-tillage system (NTS); T2,
rotation of soil cover crops (winter), and biennial onion in NTS;
T3, rotation of maize, winter grasses, and onion in NTS; T4,
succession of summer legume and annual onion in NTS; T5, rotation
of summer grass, winter grasses, and annual onion in NTS; T6,
succession of summer legume, winter grass, and annual onion in NTS;
T7, succession of maize and onion in conventional tillage system
(CTS); T8, succession of intercrops of soil cover crops (summer)

          [OC.sub.P] (g [kg.sup.-1)

       Soil bulk   Aggregates    CV%

             0.0-5.0 cm layer

T1      l0.64Ba      7.54Cb     10.60
T2      10.66Ba      9.23Ba     7.86
T3      12.88Aa     12.72Aa     11.63
T4      9.66Bb      11.29Aa     12.26
T5      10.36Bb     11.54Aa     5.96
T6      13.55Aa     11.40Ab     7.38
T7      7.90Ca       3.79Db     16.19
T8      10.88Bb     12.05Aa     11.47
CV%      8.72        12.29

             5.0-10.0cm layer

T1      6.89Aa       7.24Ca     6.22
T2      7.40Aa       5.57Da     19.19
T3      6.01Ab       9.22Aa     8.89
T4      7.30Aa       2.81Eb     9.64
T5      7.28Aa       2.95Eb     11.15
T6      6.33Aa       5.50Da     7.39
T7      6.23Aa       3.27Eb     13.54
T8      5.95Ab       9.02Ba     11.00
CV%      11.98       10.45

            10.0-20.0 cm layer

T1      4.99Aa       3.28Bb     12.08
T2      4.69Aa       3.48Bb     15.13
T3      5.01Aa       5.91Aa     15.29
T4      4.69 a       2.51Bb     15.02
T5      4.67Aa       3.08Bb     6.22
T6      4.69Ab       5.83Aa     8.08
T7      4.90Aa       3.39Bb     5.97
T8      4.65Ab       5.51Aa     2.01
CV%      10.78       13.40

            OCM (g [kg.sup.-1)

       Soil bulk   Aggregates   CV%

              0.0-5.0 cm layer

T1      22.28Bb     23.93Ba     4.20
T2      23.71Bb     25.07Ba     8.13
T3      21.25Cb     26.96Aa     8.01
T4      19.97Ca     20.62Ca     4.16
T5      20.60Cb     23.31Ba     5.89
T6      29.94Aa     22.48Bb     5.49
T7      17.06Db     22.66Ba     5.88
T8      22.98Ba     18.42Cb     5.79
CV%      6.02         6.54

             5.0-10.0cm layer

T1      18.80Ba     18.16Ca     5.21
T2      20.09Aa     20.69Ba     5.70
T3      20.07Aa     16.47Db     5.83
T4      20.80Ab     23.62Aa     5.04
T5      20.19Ab     24.75Aa     7.01
T6      18.82Bb     21.23Ba     3.80
T7      14.42Cb     21.74Ba     5.75
T8      20.62Aa     16.13Db     6.97
CV%      6.20         5.37

             10.0-20.0 cm layer

T1      15.83Cb     18.65Aa     6.65
T2      20.55Aa     19.68Aa     3.71
T3      18.38Aa     17.00Ba     8.07
T4      17.48Bb     21.24Aa     5.56
T5      19.07Ab     20.53Aa     4.94
T6      19.70Aa     17.35Bb     5.29
T7      15.77Cb     20.03Aa     4.02
T8      18.18Ba     15.80Bb     8.46
CV%      4.35         6.32

Table 6. Particulate organic nitrogen ([ON.sub.P]) and
mineral-associated organic nitrogen ([ON.sub.M]) in soil bulk and
soil aggregates of a Humic Cambisol subjected to no-tillage and
conventional tillage systems with onion crops using crop rotations
and successions. Ituporanga SC, Brazil

Means followed by the same uppercase letter in the column did not
differ significantly between treatments for soil bulk and
aggregates by the Scott-Knott test at 5%. Means followed by the
same lowercase letter in the row did not differ significantly
between soil bulk and aggregates for each treatment by the
Student's t-test (l.s.d.) at 5%; CV, coefficient of variation. T1,
succession of onion, and maize in no-tillage system (NTS); T2,
rotation of soil cover crops (winter), and biennial onion in NTS;
T3, rotation of maize, winter grasses, and onion in NTS; T4,
succession of summer legume and annual onion in NTS; T5, rotation
of summer grass, winter grasses, and annual onion in NTS; T6,
succession of summer legume, winter grass, and annual onion in NTS;
T7, succession of maize and onion in conventional tillage system
(CTS); T8, succession of intercrops of soil cover crops (summer)

          [ON.sub.P] (g [kg.sup.-1])

       Soil bulk   Aggregates    CV%

               0.0-5.0cm layer

T1      0.30Db       0.45Aa     14.11
T2      0.37Ca       0.34Ba     16.04
T3      0.31Da       0.31Ba     13.75
T4      0.48Ba       0.28Ba     21.06
T5      0.38Ca       0.25Bb     8.70
T6      0.78Aa       0.48Ab     6.85
T7      0.16Ea       0.06Db     7.93
T8      0.49Ba       0.13Cb     16.94
CV%      13.21       13.68

              5.0-10.0 cm layer

T1      0.15Ca       0.08Db     12.75
T2      0.17Ba       0.08Db     11.53
T3      0.13Ca       0.12Ca     12.18
T4      0.16Ca       0.11Cb     14.73
T5      0.20Aa       0.09Db     9.25
T6      0.20Aa       0.10Cb     14.96
T7      0.10Db       0.19Aa     5.99
T8      0.14Cb       0.16Ba     12.41
CV%      11.28       12.88

             10.0-20.0 cm layer

T1      0.07Ba       0.08Ca     13.98
T2      0.08Ba       0.05Db     15.43
T3      0.05Cb       0.07Ca     14.73
T4      0.04Cb       0.05Da     7.64
T5      0.09Ba       0.08Ca     17.39
T6      0.09Bb       0.16Aa     10.33
T7      0.13Aa       0.09Bb     9.78
T8      0.08Ba       0.10Ba     15.98
CV%      12.41       13.89

        [ON.sub.M] (g [kg.sup.-1])

       Soil bulk   Aggregates   CV%

             0.0-5.0cm layer

T1      2.98Ca       2.25Bb     3.25
T2      3.36Ba       2.78Ab     4.84
T3      2.70Ca       2.96Aa     9.47
T4      3.04Ca       2.47Bb     3.82
T5      2.94Ca       2.85Aa     6.37
T6      3.88Aa       2.81Ab     6.18
T7      2.14Da       2.20Ba     4.48
T8      2.97Ca       2.82Aa     8.75
CV%      4.36         8.38

           5.0-10.0 cm layer

T1      2.49Ba       1.99Ab     3.10
T2      2.57Ba       1.99Aa     6.16
T3      2.54Ba       2.11Ab     2.82
T4      2.62Ba       2.03Aa     5.30
T5      2.69Ba       2.12Ab     3.06
T6      2.79Aa       2.06Ab     3.89
T7      1.91Ca       2.02Aa     5.76
T8      2.66Aa       1.93Aa     8.15
CV%      4.77         5.37

             10.0-20.0 cm layer

T1      1.73Aa       1.54Ab     4.85
T2      1.77Aa       1.58Aa     6.79
T3      1.83Aa       1.58Ab     5.21
T4      1.70Ba       1.59Aa     8.82
T5      1.85Aa       l.67Ab     4.72
T6      1.83Aa       1.51Ab     3.63
T7      1.60Ca       1.65Aa     9.61
T8      1.79Aa       1.48Ab     5.21
CV%      4.20         8.29
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Author:Ferreira, Lucas Borges; Loss, Arcangelo; Giumbelli, Lucas Dupont; Ventura, Barbara Santos; Souza, Mo
Publication:Soil Research
Article Type:Report
Date:Nov 1, 2018
Words:10379
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