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Fate of N in soil amended with [sup.15]N-labelled residues of winter cereals combined with an organic N fertiliser.

Introduction

Organic farming systems rely on ecologically based practices and the fundamental components and natural processes of ecosystems. Soil organism activities, nutrient cycling and species distribution and competition are used directly and indirectly as farm management tools. In this context, soil fertility and crop nutrients are managed through tillage and cultivation practices, crop rotations, cover crops and the addition of manure, composts, crop waste material and organic fertilisers. The use of hardy winter cereals as cover crops with leguminous species or alone is a common practice that allows conversion of the mobile soil nitrate-N into immobile plant proteins, thus reducing the risk of N leaching during the autumn (fall) and winter seasons. Barley, rye or triticale used as cover crops make it possible to suppress weeds in several crops, including soybean, com and sugar beet, with a strong allelopathic potential (Moore et al. 1994; Nagabhushana et al. 2001; Dhima et al. 2006). Cropping of wheat in apple orchard soil before replanting the site with apple provided control of fungal pathogens (Mazzola 2007; Mazzola and Manici 2012). Therefore, the management of cover crops and their residues (CRs) is a very important tool to maintain soil health and fertility. The quantity and quality of CRs that return to the soil affect the build up of soil organic matter (SOM) and improve long-term soil fertility. The biochemical composition or quality of CRs affects their rate of decomposition, the timing of nutrient release and the quantity of N immobilisation (Heal et al. 1997). The C : N ratio is the most commonly used index to predict residue decomposition and N release. Net mineralisation and a rise in mineral N levels are expected with a low C: N ratio, which is therefore associated with high residue quality, whereas net immobilisation that reduces mineral N in the soil is usually observed with a high C: N ratio, associated with low residue quality. However, this index alone may not be sufficient to predict N behaviour, because the presence and amount of recalcitrant C may reduce the rate ofN mineralisation (Vigil and Kissel 1991; Quemada and Cabrera 1995). In general, the amount of lignin and polyphenols influences the decomposition of and nutrient release from CRs in the both medium and long term (Rasse et al. 2005; Machinet et al. 2011a, 2011b), and polyphenols can bind with proteins, thereby temporally immobilising N (Melillo et al. 1982; Palm and Sanchez 1991). However, lignin can be degraded from the beginning of decomposition when it is present in the cell wall as non-condensed lignin (Machinet et al. 2011a, 2011b) and polyphenols of low molecular weight are easily used by the microbial biomass (Schmidt et al. 2013).

The addition of another N source (i.e. mineral or organic fertiliser) can further alter the rate of the processes driving CR decomposition and N release. The effect of mineral N added together with CRs depends on the N content of the CRs that defines their quality. With high-quality CRs, the effect on decomposition is minimal because N is not a limiting factor for microbial growth and activity, whereas with low-quality CRs N is generally limited, thus the addition of N fertiliser may increase microbial decomposition.

Unlike a mineral N fertiliser, in which the N is immediately available, an organic N fertiliser must be decomposed by soil micro-organisms before its N becomes accessible. The N availability depends on the chemical characteristics of the organic fertiliser, such as its N content, C : N ratio and the presence and amount of easily decomposable C compounds. This last aspect is particularly important because an organic N fertiliser supplies a source of energy to the soil microbial biomass and therefore its effect on the decomposition of CRs can be different from that of a mineral N fertiliser. In organic farming, only organic N fertilisers are allowed. However, there are very few reports in the scientific literature regarding the time-dependent N release from these fertilisers and their effect on the processes mediated by the microbial biomass.

The N made available during CR decomposition, if not taken up by plants, risks being lost through denitrification or leaching unless it is stabilised in new organic matter (OM) incorporated in soil aggregates. The new formation and stabilisation of aggregates are both favoured by the addition of CRs, thus increasing the protection of SOM against decomposition (Tisdall and Oades 1982; Jastrow and Miller 1998). More specifically, CRs provide particulate PM (POM), which acts as a nucleation for the formation of macroaggregates. Macroaggregate formation and breakdown (i.e. macroaggregate turnover) occurs as CRs decompose and affect short-term nutrient cycling and long-term stabilisation of residue N in SOM (Six et al. 2000a, 20006; Plante and McGill 2002). It has been reported that residue quality affects aggregate turnover, which increases with high-quality residue and N fertiliser (Six et al. 2001). A faster macroaggregate turnover is also associated with faster mineralisation of C, nutrients and their potential loss from the system (Haynes and Beare 1997; Six et al. 2001; Vanlauwe et al. 2002). Therefore, the long-term stabilisation of residue N in SOM physically protected or chemically associated with the silt and clay fraction depends on residue quality, but the addition of another N source, such as an organic N fertiliser, may have an important effect on these processes. Supplying additional N and C sources, the organic fertiliser is expected to stimulate the rate of decomposition of low-quality residue, C mineralisation and N cycling in soil, thus potentially altering the long-term stabilisation of residue.

In the present study, the fate of N derived from the decomposition of two lsN-labelled winter cereals, used as cover crops, with or without the supply of an organic N fertiliser was followed over 1 year in a field experiment. We hypothesised that combining organic N fertiliser and CRs would produce benefits in terms of release and stabilisation in soil aggregates of N from the residues depending on residue quality. We also tested whether the losses of N from the residues in the system were consequently affected.

Material and methods

Experimental design

The study was performed in the north of Italy (Trentino-Alto Adige), which is an area mainly dedicated to organic apple orchard production (altitude 220m; 46[degrees]22'59"N, 11[degrees]17'18"E). This region receives a mean annual rainfall of 810 mm, mainly concentrated in the spring and summer. The mean summer temperature is 21.3[degrees]C and the mean winter temperature is 1[degrees]C.

The experiment was performed over 1 year on an organic apple orchard farm with mesocosms made of polyvinyl chloride tubing (10 cm internal diameter), closed at the bottom with a metallic net (3 mm mesh) and buried in the soil to a depth of 15 cm, leaving 10 cm above the soil surface to avoid overflow of rainwater. The mesocosms were arranged in the area between tree rows (at 50-cm intervals) in a completely randomised block design, with three blocks per treatment, three replicates and five destructive sampling times at 1,3, 6, 9 and 12 months (May 2011-May 2012).

The soil (Fluvic Cambisol) used to fill the mesocosms (400 g dry basis) was sampled from the 0-30 cm layer of the organic apple orchard, sieved at 4 mm and kept at 4[degrees]C before analysis. The main physicochemical characteristics of the soil were as follows; 870 g sand [kg.sup.-1] soil, 40g silt [kg.sup.-1] soil, 90 g clay [kg.sup.-1] soil; 90 g total CaC[O.sub.3] [kg.sup.-1] soil; [pH.sub.w] 7.1; soil organic carbon (SOC) 13g[kg.sup.-1]; and total nitrogen (TN) 1.4g[kg.sup.-1].

Two winter cereals were used as CRs, namely barley (Hordeum vulgare L. cv. Tidone) and triticale (Triticosecale cv. Oceania), and hydrolysed leather was the organic N fertiliser allowed in organic farming (EC Regulation No. 889/2008, http:// cur-lex.europa.eu/legal-contcnt/EN/TXT/?uri=celex%3A32008 R0889). This fertiliser is obtained through hydrolysis of vegetable tanned leather scrap at 160-165[degrees]C for at least 10 min and then dehydrated (EC Directive 96/449, http://curlex.europa.cu/legal-content/EN/TXT/HTML/?uri=CELEX:319 97D0735&from=IT). The experimental design included a control soil (S), soil with barley residues (S+B), soil with barley residues and hydrolysed leather (S+B+L), soil with triticale residues (S+T) and soil with triticale and hydrolysed leather (S+T+L). Barley and triticale were grown in the greenhouse and fertilised with [sup.15]N-labcIlcd [(N[H.sub.4]).sub.2]S[O.sub.4] solution (7.5% atom [sup.15]N). At the barrel stage, plants were harvested and cut into 1- to 2-cm pieces (leaves and stem pieces were thoroughly mixed in order to obtain an even composition of the two plant parts) and mixed with the soil at a rate equivalent to 10[tha.sup.-1] fresh material. Hydrolysed leather (50% C, 13% N), hereinafter simply 'leather', was added as a powder at a rate equivalent to 50 kg N [ha.sup.-1]. The mesocosms were not cropped in order to avoid crop root interference with C, N and aggregate dynamics.

Residue analyses

The main structural characteristics of the barley and triticale residues were investigated by simultaneous thermogravimetric and differential thermal analysis (TG-DTA 92B; Setaram, Lyon, France; Table 1). Approximately 5 mg finely ground dry samples was first heated isothermally to 30[degrees]C for 10 min (2[degrees]C [min.sup.-1]) and subsequently heated from 30[degrees]C to 800[degrees]C (10[degrees]C [min.sup.-1]) in dynamic air with a flow rate of 130mL [min.sup.-1] (Montecchio et al. 2006). The structural sample composition is strictly related to the mass loss and the enthalpy changes during its heating. DTA curves (data not shown) of barley and triticale showed three main reactions: one endothermic, related to dehydration of the sample (~8% for both cereals) and two exothermic reactions with a labile peak at low or medium temperatures and a recalcitrant one at high temperatures. The labile peak is due to both the oxidation of carbohydrates, such as cellulose, and the loss of carboxyl groups, whereas the second peak is attributed to thermally more stable aromatic structures, including lignin (Rovira et al. 2008). The mass loss related to these two peaks was used to quantify carbohydrates and aromatic compounds of barley and triticale (Table 1).

The added residues were carefully recovered from the soil at each sampling date, dried at 60[degrees]C and weighed to calculate mass loss during the degradation process. Then, the dry residues were finely ground and analysed for their C and N content (elemental analyser EA 1110; Thermo Scientific, Waltham, MA, USA) and [sup.15]N abundance (Delta Plus; Thermo Scientific).

Soil analyses

At the last sampling date, after residue recovery, soil samples were air dried. One aliquot was ground and analysed for total C and N and 1SN enrichment, whereas another aliquot (5 g) was used for physical fractionation. Aggregates were separated by wet sieving the soil on two sieves, according to the method of Elliott (1986) with slight modifications. Briefly, the soil sample was evenly spread on top of the 250-[micro]m sieve, submerged in deionised water at room temperature for 10 min and subsequently sieved to separate water-stable aggregates by moving the sieve in an up-and-down motion with 50 repetitions over a period of 20 min. Three soil aggregate fractions were recovered: >250 [micro]m (macroaggregates; macro); 53-250 [micro]m (microaggregates; micro); <53 [micro]m (silt and clay, s+c). Once dried, the whole soil and the aggregate fractions were weighed and analysed for total C and N and [sup.15]N abundance by Continuous Flow-Isotope Ratio Mass Spectrometry (CF-IRMS) (Delta Plus; Thermo Scientific). In the soil and in each aggregate class, the fraction (f) of N derived from barley or triticale residues (NDFR) was calculated as follows:

f(NDFR)% = [sup.15]N excess soil/[sup.15]N excess residues x 100

where [sup.15]N excess is the [sup.15]N atom % of the soil or residues minus the [sup.15]N atom % at natural abundance.

The quantity of N released from labelled residues ([N.sub.new]) derived from labelled residues in the whole soil and in the fractions was then calculated as follows:

[N.sub.new] = f x [N.sub.t]

where [N.sub.t] is the total N content measured in the sample.

It follows that the portion (p) of N from residues and recovered in soil and fractions as [N.sub.new] can be calculated as follows:

p = [N.sub.new]/[N.sub.res]

where Nres is the total amount of N initially added with the residues.

Statistical analysis

Data for the degradation of residues were analysed as in a factorial design with three factors: time (five levels), residues (two levels) and leather N (two levels). Significant differences (P [less than or equal to] 0.05) between means were determined using the Student-Newman-Keuls' test. When the interaction between time, residues and leather N ([N.sub.leather]) was significant, twice the standard error of the mean was used as the minimum difference between two significantly different mean values (Saville and Rowarth 2008). Data for soil N, macro, micro, s+c and N in each fraction were analysed as in a factorial design with an external control (S) and two factors, namely residue (two levels: barley (B) and triticale (T)) and N leather (two levels: 0 kg [N.sub.leather] [ha.sup.-1] (ON) and 50 kg [N.sub.leather] [ha.sup.-1] (50N)). Effects of treatments (S vs all), residue (B vs T), N leather (ON vs 50N) and the interaction between residue and N leather (residue x N leather) were determined with orthogonal contrasts.

Results

Loss of mass and C mineralisation

Barley and triticale showed very different degradation patterns (Fig. la). After 1 month of incubation, the mass loss of barley corresponded to 31 % of the initial amount, whereas no mass loss was detected for triticale. The mass loss of barley continued throughout the experiment and, after 1 year, almost 77% of the barley residues was degraded. The mass loss of triticale started only after the first month of incubation; approximately 20% of the initial mass was lost between the first and third months, and from then, until 9 months of incubation, no further mass loss was recorded. However, in the last 3 months triticale was significantly degraded and the mass loss measured at the end of the year was almost 50% of that initially added.

The addition of leather had different effects on the mass loss of the two species. The degradation of barley residues was partially slowed throughout the incubation period, whereas the degradation of triticale was significantly increased by the presence of leather, and this resulted in a mass loss similar to that of the barley residues. At the end of the year, 72% of the barley was degraded in the S+B+L treatment, compared with 64% of triticale in the S+T+L treatment.

The amounts and patterns of C mineralisation were also different for the two plant species (Fig. 1 b). At the end of incubation, C loss from barley residues was equal to 83% of the initial amount, and most of it (57% of the initial amount) was lost during the first month of incubation. A significantly lower amount of C was lost from triticale (63% at the end of the incubation), with only 20% lost after the first month. The presence of leather significantly increased the C mineralisation of triticale, which reached 75% of the initial amount, and significantly decreased the C mineralisation of barley.

Loss of N

As observed for both mass and C loss, the amounts and patterns of N release were also significantly different between the two plant species (Fig. 2a). Approximately 75% of the N added with barley residues was released by the end of the incubation, and almost 45% of the residues N was mineralised during the first month. The release of N from triticale residues was significantly lower, only reaching 38% of the initial amount at the end of the year; in addition, it only started after the first 3 months of incubation. The presence of leather significantly increased N release from triticale from the beginning of the incubation, but reduced N release from barley even though this reduction was not significant at the end of the incubation.

The loss of [sup.15]N was also different between the two plant species (Fig. 2b). The [sup.15]N release from barley residues was 83% of the initial amount at the end of the year, significantly higher than the 57% released from triticale residues. The presence of leather also affected [sup.15]N release, increasing it from triticale and reducing it from barley. Flowever, for all the treatments the amount of [sup.15]N released was higher than that of total N.

Soil total N and residue-derived N

At the end of the incubation, only the presence of leather caused a significant increase in soil N compared with the soil with the residues alone (Table 2). Of the soil total N, the amount derived from residues ranged from 5.5% in the S+T+L treatment to approximately 7.0% in the other treatments (Table 3). These values allowed us to calculate the portion (p) of N added with barley or triticale recovered in soil at the end of the year, which was similar in both the treatments and ranged between 56% of barley and 64% of triticale (Table 3).

Soil aggregate fractions

The soil physical fractionation showed that macro and micro together accounted for more than 90% of the soil dry weight. whereas s+c contributed only 5% 8% (Table 2). All the treatments induced a significant increase in the amount of macro and a significant decrease in the amount of micro compared with the control. In contrast, no effect was observed for the s+c. In the different treatments, the addition of leather caused a significantly higher accumulation of macro and a decrease in micro and s+c compared with the soils where only residues were added.

N in the aggregate fractions

The N content differed among the three fractions (Table 2). Despite the treatments, the highest N content was measured in the s+c, the lowest was measured in the micro and intermediate values were measured in the macro. All treatments significantly increased the N content of macro and s+c compared with control; a similar trend was observed for micro, although the difference was not significant. The total amount of N in the fractions, calculated on the basis of their relative weight, followed the order macro > micro > s+c, and this trend was observed for all treatments. The presence of leather significantly increased the total amount of N in the macro and significantly decreased it in the s+c (Table 2).

The percentage of NDFR was similar in the three fractions, but was significantly affected by the addition of leather, which reduced NDFR compared with S+B and S+T (Fig. 3a). In contrast, the percentage of residue N recovered in the fractions was different among the three fractions, because it was affected by their relative weight in the soil (Fig. 3b). The highest portion of residue N was recovered in the macro, the lowest in the s+c and an intermediate amount in the micro. The addition of leather also reduced the portion of residue N compared with the residues alone, particularly in the micro and s+c fractions.

Budget of residue-derived N

The isotopic approach made it possible to calculate a complete budget of the residue-derived N (Fig. 4). The difference between the initial amount of N added to soil and that recovered in the soil and in the undecomposed residues at the end of the incubation was the amount not accounted for and presumed to be lost from the system. The greatest amount of residue N lost was measured in the S+B (27.6%), whereas no residue N losses were measured in the S+T. The addition of leather had an opposite effect on N losses, reducing those from S+B (17.5%) and increasing those from S+T (18.4%).

Discussion

Residue decomposition and N release

The decomposition rate and N release of CRs under the same environmental conditions depends on the quality of residues, which is defined by their biochemical characteristics, in particular the N content and the C : N ratio, but also the presence and amount of polyphenols, lignin or other recalcitrant C compounds (Vigil and Kissel 1991; Quemada and Cabrera 1995). Barley residues underwent fast decomposition and N release, as expected on the basis of their high N content and low C : N ratio. The slow degradation rate of triticale residues was probably the result not only of a higher C : N ratio, but also of the biochemical characteristics of C components. Triticale residues were actually characterised by a higher recalcitrance because of their higher content of thermostable aromatic components and consequent higher values of the aromatic: carbohydrate ratio, and the aromatic: N ratio (Table 1).

The presence of another organic N source (leather) had a positive effect on decomposition and N release of triticale residues and no effect or a negative effect on barley residue degradation. The effect of mineral N fertiliser addition on the decomposition of residues of different quality, studied by others (Sail et al. 2003; Gentile et al. 2008), was found to stimulate early C mineralisation of low-quality residues but inhibited this process with higher-quality residues. The effect of the organic N fertiliser on N release was, instead, different from what was observed by Sail et al. (2003) and Gentile et al. (2008). These authors found that the addition of mineral N fertiliser induced a greater release of N from high-quality residues and higher N immobilisation with low-quality residues. However, Gentile et al. (2008) also observed a shift from a negative to positive interactive effect on crop yield and N uptake as a consequence of increased N availability, when residues of decreasing quality were combined with mineral fertiliser. This behaviour, according to Gentile et al. (2008), was the consequence of the effect of fertiliser addition on the reduced leaching losses due to a greater N immobilisation, and on the stimulation of N release from low-quality residues.

The CRs studied by Gentile et al. (2008) differed considerably in terms of quality compared with those used in the present study. Moreover, the N supply in the present study was organic in form. Leather is both an energy and nutrient source for the microbial biomass, characterised by a very narrow C : N ratio (4). Therefore, it could have been a preferred substrate for the microbial biomass compared with the added residues. This preferential utilisation of leather at the beginning of the incubation slowed the degradation of barley because the micro-organisms had already received energy and nutrients from the fertiliser for their growth, whereas the degradation of triticale was stimulated because the use of the leather as a substrate favoured microbial growth and activity and the synthesis of enzymes able to degrade a more recalcitrant substrate.

During residue decomposition, particularly in the first month, the release of [sup.15]N was higher than the loss of total N. A significant change in residue labelling, which decreased from 6.1217 atom % [sup.15]N of triticale and 5.7690 atom % [sup.15]N of barley to approximately 4.4 atom % l 5N for both species, was also observed in the same period. This variation in isotopic composition of the residues was probably the result of added N interaction because of the pool substitution process, where some of the [sup.15]N-labelled N was released from degrading residues and partially substituted by soil-derived N immobilised in the microbial biomass proliferating on the decomposing residues. The pool substitution process is actually reported to occur when sufficient quantities of easily decomposable OM are available in soil, or when fresh plant residues are added (Azam 2002), because these conditions favour the immobilisation of considerable quantities of N, even if a net mineralisation is observed (Broadbent and Nakashima 1974; Shen et al. 1984).

Soil N stabilisation

At the end of the year, the treatments to which only residues were added did not result in a significant increase in total soil N. The application of leather, as an organic N fertiliser, together with residues led to an increase in total soil N compared with the residues alone. This behaviour differed from that observed by Chivenge et al. (2011) in a sandy soil, where the combined application of residues and mineral fertiliser did not alter soil N content compared with residues alone. However, the extra amount of total soil N we measured did not derive from an accumulation in soil of residue-derived N because this portion was similar for all the treatments, despite the different residue decomposition patterns. Hence, we hypothesised that this extra N was due to the presence of an amount of slow-releasc N in leather (Govi et al. 1995).

The combined addition of residues and leather also affected the relative distribution of soil aggregates and the stabilisation of residue N in the different physical fractions compared with the control and residues alone. An increase in macro, and a decrease in the finer fractions (micro and s+c) was actually found in the presence of leather. This positive effect on macroaggregate formation differed from what is usually found with the combined application of crop residues and mineral N fertiliser. Indeed, mineral N applied with residues enhances mineralisation and macroaggregate turnover (Bossuyt et al. 2001), whose effect is a lower mean weight diameter of soil fractions and a lower SOC compared with residues applied alone (Graham et al. 2002; Blair et al. 2005). The immediate availability of N from mineral fertiliser actually leads to a favourable condition for the growth and activity of the microbial biomass, the increase in mineralisation processes and the consumption of SOC by soil micro-organisms. Instead, leather contains both C and N in an optimal ratio for the microbial biomass, which can easily use this substrate as a source of energy and nutrients for its growth and activity. In this context, less SOC is used by micro-organisms or even new OM is built up and accumulated through greater aggregation. This C sequestration and stabilisation was found to be associated with an increase in the mean weight diameter that allowed a greater protection of C associated with the microaggregates formed within small macroaggregates (Kong et al. 2005; Gentile et al. 2010).

The percentage of NDFR and the portion of residue N recovered in the three aggregate fractions were affected by the addition of leather, which significantly decreased the amount of N derived from residues in the fractions, particularly in the finest ones. These results are only in partial agreement with those of Chivenge et al. (2011), who found a smaller stabilisation of low-quality residues N in the presence of mineral N fertiliser but no effect on N stabilisation when fertiliser was added to high-quality residues. However, this effect was observed only in a clayey soil, and not in a sandy soil. More recently, Gentile et al. (2013) found that the combined application of residues and mineral N fertiliser reduced residue N stabilisation, but increased fertiliser N stabilisation. These authors also calculated that the increase in fertiliser N stabilised generally outweighed the loss of residue N in the combined input treatment. Gentile et al. (2013) suggested that the reduced stabilisation of residue N in the aggregates was not due to an increased mineralisation of residues, but to a greater retention of fertiliser N by soil micro-organisms that may have preferentially consumed and further stabilised within the soil aggregates this readily available N. In the present study, we cannot exclude the possibility that an increased aggregate turnover stimulated by leather would have caused the release of already stabilised residue N and its substitution with leather N. However, the greater macroaggregate formation observed with leather and the lower residue N retention found in the finest and more stable fractions seem to support the mechanism proposed by Gentile et al. (2013). Because the addition of leather affected both the release of residue N and its stabilisation, it also consequently affected the amount of residue N not accounted for and presumed to be lost. We found that the faster the residue mineralisation, the higher the amount of residue N lost from the system, mainly through leaching considering the light texture of the soil and the environmental conditions not favourable for denitrification. In agreement with the results reported by others (Sakala et al. 2000; Gentile et al. 2009), the greatest losses of residue N occurred with high-quality residues, whereas no losses were seen with low-quality residues because they released less N, which was probably immobilised. The addition of a mineral N fertiliser is often reported to increase the N losses when added to high-quality residues and, in contrast, to reduce them when added to low-quality residues as a result of a greater N immobilisation (Sakala et al. 2000; Gentile et al. 2009).

In the present study, the addition of leather had an opposite effect because the losses of residue-derived N were reduced with high-quality residues and increased with low-quality residues. Leather added to barley reduced the mineralisation of residue N during the first phase of degradation when most of the residue N is quickly released, probably because the narrow C : N ratio of leather makes it a preferred substrate for the microbial biomass compared with the residue. This reduced accumulation of residue N also reduced the leaching losses. In contrast, when added to low-quality residues, leather stimulated the degradation and the release of residue N from the early decomposition phase, allowing part of this residue N to be lost by leaching.

Conclusions

The effect of leather as an organic N fertiliser combined with crop residues on their mineralisation and N dynamics is driven by the capacity of leather to supply not only nutrients, but also energy to micro-organisms. The leather affected the release of residue N depending on the residue quality, reducing the availability of barley N and increasing that of triticale. However, the stabilisation of residue N in the finest aggregate soil fractions was always reduced by leather. These results are of special interest in organic farming where knowledge of the mechanisms leading to N availability and storage in soil is crucial to achieve sustainable production.

http://dx.doi.org/10.1071/SR15055

Acknowledgements

This work was funded by the Italian Ministry of Agricultural Food and Forestry Policies (MIPAAF), 'Project Increase of soil endogenous functionality in organic orchards: cereal cover crops to increase microbial components involved in soil repressiveness against root pathogens which are agents of production decline--ENDOBIOFRUIT' (DM 24318/7742/09). The authors thank Graziella Marcolini for her assistance with this study.

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Paola Gioacchini (A,D), Daniela Montecchio (A), Emanuela Gnudi (A), Valeria Terzi (B), Antonio Michele Stanca (C), Claudio Ciavatta (A), and Claudio Marzadori (A)

(A) Department of Agricultural Sciences - Alma Mater Studiorum University of Bologna. Viale Fanin 40, 40127 Bologna, Italy.

(B) CRA-GPG, Via San Protaso 302, 29017 Fiorenzuola d'Arda, Italy.

(C) Department of Agriculture and Food Sciences, University of Modena and Reggio Emilia, Via G. Amendola 2, 42122 Reggio Emilia, Italy.

(D) Corresponding author. Email: paola.gioacchini@unibo.it

Received 19 February 2015, accepted 20 July 2015, published online 15 March 2016

Table 1. Main characteristics of barley (Hordeum vulgare L. cv.
Tidone) and triticale (Triticosecale cv. Oceania) used in the
present study

            Humidity   C (%)   N (%)   Atoms [sup.15]N   C : N ratio
              (%)                            (%)

Barley         85      40.88   1.70        5.7590            24
Triticale      80      42.34   1.47        6.1217            29

            Aromatic: carbohydrates   Aromatic: N
                     ratio               ratio

Barley               0.39                14.41
Triticale            0.44                17.89

Table 2. Total soil N, weight of aggregate fractions
(macroaggregates (macro), microaggregates (micro), silt and clay
(s+c)), aggregate N content and partitioning of soil total N in
aggregate fractions at the end of 1 year incubation with residues
alone or combined with leather

* P [less than or equal to] 0.05; ** P [less than or equal to]
0.01; *** P [less than or equal to] 0.001; NS, not significant. S,
control soil; S+B, soil with barley residues; S+B+L, soil with
barley residues and hydrolysed leather; S+T, soil with triticale
residues; S+T+L, soil with triticale residues and hydrolysed
leather. 0N, 0kg leather N [ha.sup.-1]; 50N, 50kg leather N
[ha.sup.-1]

                           N leather              N
                        (kg [ha.sup.-1])   (g [kg.sup.-1])
Treatments
  S                            0                1.40
  S+B                          0                1.41
  S+B+L                        50               1.51
  S+T                          0                1.47
  S+T+L                        50               1.53
Orthogonal contrast
  S vs all                                       NS
  B vs T                                         NS
  0N vs 50N                                       *
  Residue x N                                    NS
  leather interaction

                           Macro (A)        Macro (B) N     Macro (C)
                        (g [kg.sup.-1])   (g [kg.sup.-1])     N (g)
Treatments
  S                          435.5             1.26           0.55
  S+B                        477.5             1.49           0.71
  S+B+L                      570.7             1.52           0.87
  S+T                        494.1             1.58           0.78
  S+T+L                      514.9             1.37           0.80
Orthogonal contrast
  S vs all                    **                ***            **
  B vs T                      NS                NS             NS
  0N vs 50N                    *                NS              *
  Residue x N                 NS                NS             NS
  leather interaction

                           Micro (A)        Micro (B) N     Micro (C)
                        (g [kg.sup.-1])   (g [kg.sup.-1])     N (g)
Treatments
  S                          484.1             1.02           0.50
  S+B                        445.3             1.17           0.52
  S+B+L                      378.2             1.14           0.43
  S+T                        424.7             1.14           0.49
  S+T+L                      417.6             1.24           0.44
Orthogonal contrast
  S vs all                    **                NS             NS
  B vs T                      NS                NS             NS
  0N vs 50N                    *                NS             NS
  Residue x N                 NS                NS             NS
  leather interaction

                            s+c (A)          s+c (B) N      s+c (C) N
                        (g [kg.sup.-1])   (g [kg.sup.-1])      (g)
Treatments
  S                          80.4              1.68           0.13
  S+B                        77.1              1.78           0.14
  S+B+L                      51.1              1.88           0.10
  S+T                        81.2              1.84           0.15
  S+T+L                      67.4              1.81           0.12
Orthogonal contrast
  S vs all                    NS                 *             NS
  B vs T                      NS                NS             NS
  0N vs 50N                    *                NS              *
  Residue x N                 NS                NS             NS
  leather interaction

(A) Weight of aggregate fractions on a dry soil basis.

(B) N content on an aggregate fraction basis.

(C) N content in aggregate fractions given by the N content on an
aggregate fraction basis multiplied by the weight of the aggregate
fraction.

Table 3. Residues-derived N (NDFR) in the soil and the portion of
N added with residues and recovered in the soil at the end of the
year

* P [less than or equal to] 0.05; NS, not significant. Values
differing by [greater than or equal to] 2 s.e.m. are
significantly different. S+B, soil with barley residues; S+T,
soil with triticale residues; [N.sub.new], the amount of N
released from labelled residues recovered in the soil;
[N.sub.res], the total amount of N initially added with the
residues

Treatment                  Leather N       NDFR    [N.sub.new]/
                        (kg [ha.sup.-1])   (%)    [N.sub.res] (%)

S+B                            0           6.9          56
                               50          7.0          60
S+T                            0           7.1          64
                               50          5.5          57
2 s.e.m.                                   0.80
  Residue                                   NS          NS
  Leather N                                 NS          NS
  Residue x leather N                       *           NS

Fig. 4. Budget of residue-derived N after 1 year of incubation in
soil showing the percentage of residue-derived N in the
undecomposed residues and soil. The difference between the initial
amount of residue N and these two components was accounted for as
residue N lost. S+B, soil with barley residues; S+B+L, soil with
barley residues and hydrolysed leather; S+T, soil with triticale
residues; S+T+L, soil with triticale residues and hydrolysed
leather.

              soil     residue     loss

(a) S+B       55.9     16.5        27.6
(b) S+T       6.40                 36.0
(c) S+B+L     60.1     22.4        17.5
(d) S+T+L     56.6     25.0        18.4

Note: Table made from pie chart
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Author:Gioacchini, Paola; Montecchio, Daniela; Gnudi, Emanuela; Terzi, Valeria; Stanca, Antonio Michele; Ci
Publication:Soil Research
Article Type:Report
Date:Mar 1, 2016
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