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Total soil organic matter and its labile pools following mulga (Acacia aneura) clearing for pasture development and cropping. 2. Total and labile nitrogen.

Introduction

About 150Mha of Australia originally supported mulgadominated (Acacia aneura F. Muell. Ex. Benth.) woodlands (Johnson and Burrows 1994). In Queensland, mulga forests originally occupied 11.2 Mha, of which 12% has been cleared (Wilson et al. 2002; Department of Natural Resources and Mines 2003). The cleared area is primarily used for pasture but small areas are also used for cropping. There is a concern for the sustainable use of cleared mulga land because of the arid to semi-arid environments (250-500 mm rainfall, often exceeding 30% annual rainfall variability) and the fragile soils (Red Kandosols and Red Tenosols), which are of low fertility and are low in soil organic matter and plant-available phosphorus (Condonet al. 1969; Mills 1986).

Dalai et al. (2005) found that land use change from mulga to pasture and to cropping led to declines in soil organic C of 2.7 and 5.0 t/ha, respectively, from the top 0.3 m. Corresponding soil C losses in the top 0.05 m depth were 28% and 35% for pasture and cropping, respectively, compared with mulga. Soil N losses due to land-use change from mulga to pasture and cropping are also expected because C and N are closely associated in soil organic matter. Dalal and Mayer (1986a) measured losses in soil total N of 28% when a red Kandosol was cleared of native vegetation and used for cereal cropping over 20 years in southern Queensland. Loss in mineralisable N (anaerobic incubation for 7 days at 40[degrees]C) was even greater (51%), indicating that the ability of the remaining organic matter to sustain plant productivity had deteriorated even more than indicated by the loss in total soil organic matter. Similarly, the light fraction (density <1.6-2Mg/[m.sup.3]) N was affected more than total soil N when land use changed from native vegetation to cropping. The light fraction pool has been used as a sensitive indicator of soil organic matter quality (Dalai and Mayer 1986b; Gregorich and Janzen 1996). However, clearing forest for pasture production may result in different trends in soil organic matter quality and quantity to those observed when land is cleared for cereal cropping (Murty et al. 2002).

Murty el al. (2002) reviewed the effect of land-use change from forest to pasture on soil N and found no significant global change. However, changes in total soil N at individual sites ranged from -50% to +20%, indicating that ecosystems may lose or gain N, depending on soil type (Harms and Dalai 2003), increased N deposition (van Breemen 2002), pasture management (grazing intensity), plant residue retention or removal (Murty et al. 2002), fertiliser applications (Fearnside and Barbosa 1998), and proportion of legume component in the pasture (Dalal et al. 1995). Because most mulga land used for pasture grazing does not receive any N fertiliser despite the continual removal of nutrients in animal produce, there is likely to be a loss in productivity and, hence, reduced soil organic matter quantity, as well as quality, in the long term (Mills 1986). However, the quantitative loss of total soil and labile N, and controls on N availability, in mulga land cleared for pasture production in southern Queensland are not known.

Biological [N.sup.2] fxation associated with trees and shrubs plays a major role in the functioning of many ecosystems, but it can be difficult to quantify the amounts of [N.sup.2] fixed (Boddey et al. 2000). Boddey et al. (2000) utilised the [sup.15]N natural abundance technique, which exploits the naturally occurring differences in [delta][sup.15]N between mineral N in the soil and atmospheric [N.sup.2] to quantify the contributions of [N.sup.2] fixation by trees. If [delta][sup.15]N of atmospheric [N.sup.2] is by definition 0 [per thousand], then [delta][sup.15]N in the tissues of plants that fix atmospheric [N.sup.2], such as legumes, should also be close to 0 [per thousand], amended by soil mineral N. Where the [delta][sup.15]N value of other plant N sources (such as mineral N in soil) is different from 0 [per thousand], a mixing model can be used to estimate the proportion of N derived from atmospheric [N.sup.2] (Robinson 2001). Thus, the [delta][sup.15]N values of mulga (a potentially atmospheric [N.sup.2]fixing legume) and non-[N.sup.2] fixing buffel pasture can be used to assess the possible input of fixed N to the mulga ecosystem (Pate et al. 1998). Buffel grass receives its N supply from soil N rather than atmospheric [N.sup.2] and thus can be used as a reference plant (Ledgard and Peoples 1988) to estimate [N.sup.2] fixed by mulga. In addition, a few Eucalyptus melanophloia (silver-leaf ironbark) trees growing among the mulga were also sampled and used as non-[N.sup.2] fixing reference vegetation.

The objectives of this study were to quantitatively assess: (i) the changes in soil total N down to 1 m depth: (ii) light fraction N and mineralisable N to demonstrate changes in soil organic matter quality down to 1 m depth; and (iii) the potential input of fixed N to the mulga ecosystem.

Materials and methods

The study site

Dalai et al. (2005) provided a detailed description of the study site and methods used. Briefly, the study site is located on the 'Mulga View' property near St George (27:59%, 148-33'E), southern Queensland, Australia. The soil type is a Red Kandosol (Isbell 1996), with a clay content of 12% and soil pH of 6.0 in the 0-0.1 m soil layer, increasing with depth to 23% clay and p[H.sub.1:5 [H.sub.2]O] 6.2 in the 0.6-1.0m soil depth (Table 1). Mean annual temperature at St George is 21[degrees]C and mean annual rainfall and pan evapouration are 516 and 1954 mm, respectively.

Prior to clearing in 1980, the whole site was under mulga-dominated (Acacia aneura) woodland or open forest. A portion of the cleared area was ploughed and sown to buffel grass (pasture area) and an adjoining portion (cropping area) was ploughed and sown to wheat (Triticum aestivum L.). The pasture area has been grazed (with varying intensity) by cattle and the cropping area has grown mostly wheat but also a few crops of sorghum. The cereal crops have usually received 20 kg/ha of monoammonium phosphate but no other fertilisers; the pastures have not received any fertilisers. The average wheat and sorghum grain yields have been about 0.8 t/ha (Bruce Scriven, pets. comm.). The uncleared mulga area carries a high-density, open mulga forest, with an estimated aboveground biomass of 50 t C/ha (Dalal et al. 2005).

The presence of charcoal at the mulga site indicates that occasional fires must have swept through the forest in the past. Griffin and Hodgkinson (1986) reported that fire use by aboriginals in the Mulga Lands was restricted to infrequent burning of many small patches. This created a mosaic of different vegetation recovery states leading to a range of niches for plants and animals and allowed for many potential responses by the biota. Fire use by the aboriginals declined early in the 20th century with the rapid occupation of the Mulga Lands by European settlers (Griffin and Hodgkinson 1986).

Soil sampling

For this study, soil samples were collected in November 2001 from the mulga and the adjoining pasture and cropping areas (located about 200 m apart). Representative soil samples were taken from each area by sampling an area 50m by 50m on a 10-m grid. The samples were taken at 0-0.05, 0.05-0.1, 0.1-0.2, 0.2-0.3, 0.3-0.6, and 0.6-1.0 m depths by a hydraulically operated sampler with a 50-mm-diameter steel tube. Five samples each from 0-0.05, 0.05-0.1. 0.1-0.2, and 0.2-0.3 m depths, and 3 samples each from 0.3-0.6 and 0.6-1.0 m depths, were combined to obtain composite samples for each respective depth increment. At each site, 5 of these composite samples were obtained for each depth. The soil samples were sealed in plastic bags in the field and stored at 4C until further analysis. Additional soil sampling was done to obtain intact soil cores for bulk density measurements.

Plant sampling

In November 2001, representative samples (3-5 replicates) of mulga twigs and phyllodes were obtained by harvesting branches at a height of approximately 2-3 m, and fine roots were obtained from the 0-0.3 m soil layer by carefully collecting and sieving soil in the field. Wheat shoots (vegetative parts only) and roots were sampled at the time of wheat harvest: buffel grass shoots and roots were sampled at the same time. Leaf samples were also obtained from Eucalyptus melanophloia (silver leaf ironbark) trees growing among the mulga. Roots were gently washed to remove adhering soil. The roots and shoots were then dried at 40[degrees]C for 48-72 h, ground to <1 mm and stored m sealed plastic containers.

Light fraction N

A modification of the method of Golchin el al. (1994) was used to separate the light fraction soil organic matter. Briefly, 10g of air-dried soil (<2 mm) was placed into 40 mL of sodium polytungstate solution (1.6 Mg/[m.sup.3] density) in a 50-mL plastic centrifuge tube. The tube was gently inverted several times by hand until the soil was just fully dispersed and then allowed to stand for 30 min before centrifuging at 2000G for 5 min. The tube was swirled gently to wash off the clay particles adhering to the stopper and tube wall, and then centrifuged for a further 10 min at 2000G. The supernatant with floated particles was decanted onto a 45-mm-diameter Millipore membrane (<3 [micro]m) under vacuum and washed with deionised water. The light fraction soil organic matter collected on the filter membrane was dried at 50[degrees]C for 48 h and weighed and the samples were transferred to, and stored in, sealed plastic containers.

Mineralisable N

Soil samples were collected for measurement of ammonium-N (N[H.sub.4.sup.+]-N) and nitrate-N (N[O.sub.3.sup.-]-N) in soil under mulga, pasture, and cropping (recently sown to buffel pasture) in October 2003 using the in sire method of measuring N mineralisation developed by Raison et al. (1987). Twelve stainless steel tubes (50mm internal diameter) in each land-use area were inserted to a depth of l m in October 2003 to exclude root N uptake: 6 tubes were removed immediately and the remaining 6 were removed in December 2003 after incubation under field conditions (uncovered tubes) for that period. Soil samples were collected from the tubes, sectioned into 0-0.1, 0.1-0.2, 0.2-0.3, 0.3-0.6, and 0.6-1.0m depths, and analysed for ammonium and nitrate N. During this period 60 mm of rain was received: however, no increased amount of ammonium or nitrate leaching was detected in the 0.6-1-m depths.

Analytical techniques

Total C and N concentrations in the fine-ground soil (<0.25mm, no gravel was found in the soil samples) and crop residue samples were determined by dry-combustion with a LECO CNS-2000 analyser (LECO Corporation, MI, USA). No carbonate was detected in the soil. Light fraction samples were analysed for C with the LECO analyser and N on an isotope ratio mass spectrometer (see below). Ammonium-N and N[O.sub.3.sup.-]-N were extracted by shaking field-moist soil for 1 h in 2M KCl (1 : 5 soil : solution). Ammonium-N (Crooke and Simpson 1971) and N[O.sub.3.sup.-]-N (Best 1976) in the extracts were determined colourimetrically on an autoanalyser (Technicon 1977).

For natural abundance [sup.15]N analyses of soil and plant materials, the analytical procedure described by Krull and Skjemstad (2003) was used. Briefly, an adequate amount (40-100 [micro]g of N) of material was placed into tin capsules and sealed. The samples were combusted and [N.sub.2] gases were separated by gas chromatography for analysis of total N as well as for [delta][sup.15]N on a 20-20 Europa Scientific Automated Nitrogen Carbon Analysis-Mass Spectrometer (ANCA-MS). The [delta][sup.15]N values were expressed as per mil ([per thousand]) deviation from [N.sub.air] standards (Peterson and Fry 1987). The [delta][sup.15]N values of plant materials collected from the site are given in Table 2.

Statistical analysis

Treatment effects were assessed using the analysis of variance (ANOVA) in GENSTAT Release 6.1 (Payne 2002). Mulga, buffel pasture, and cropping plots were the main treatment plots, and depths were the subplots in a split-plot design for soil samples. Treatment means were compared using the least significant difference (l.s.d.) test at P = 0.05.

Results and discussion

Total soil nitrogen

Total soil N concentrations were lower by 28% and 40% in the 0-0.05 m depth under pasture and cropping, respectively, compared with the soil under adjacent uncleared mulga (Table 3). Soil N was also lower under cropping than under mulga in the 0.05-0.1 m and 0.2-0.3m depths, and was lower under pasture than under mulga in the 0.05-0.1 m depth. Murty et al. (2002) summarised the changes in total soil N from forest to pasture and found the values ranged from -50% to +320%, whereas conversion from forest to cultivated land had values ranging from -50% to +20%. A decrease in N content occurred in those soils initially high in C and N, whereas N increases were attributed to low initial C and N content, an increase in legume component, fertiliser application, and low-intensity grazing. Thus, where soil C losses were observed, N losses also occurred, similar to results obtained in the present study (Dalai et al. 2005).

Dalai et al. (2005) reported that soil bulk density was significantly increased due to clearing of mulga for pasture and crop production; therefore, the amounts of soil N in equivalent soil mass under mulga, pasture, and cropping were calculated (Table 4). Cumulative soil N was significantly lower under cropping (by 340 kgN/ha) than under mulga down to a depth of 1 m, while a significant decrease of 240 kg N/ha occurred in cumulative soil N under pasture down to a depth of 0.3 m. Under both pasture and cropping, most of these decreases in soil N occurred in the top 0.1 m, similar to the trends observed for soil organic C (Dalal et al. 2005). This agrees with Harms and Dalai (2003), who also measured soil N losses of 235kg N/ha from 38 grazing sites and 289 kgN/ha from 11 cropping sites in south and central Queensland.

The estimated rate (k) of loss of total N (k = [ln ([N.sub.t]/[N.sub.0])/20], where [N.sub.t] and [N.sub.0] are soil total N at time t (20 years) and under mulga, respectively) from the top 0.05m depth varied from 0.017/year under pasture to 0.026/year under cropping, with a turnover period (1/k) of 60 and 38 years, respectively. These values are comparable to but higher than mulga [C.sub.3]-C loss (Dalai et al. 2005), possibly due to N immobilisation. Similar to organic C, the rate of loss decreased and turnover time of soil total N increased with depth, as expected (Dalal and Mayer 1986c).

Total N loss down to 1 m depth in the cropped soil, on average, was 17 kg N/ha.year. This was much lower than that reported by Dalai and Mayer (1986c) for 6 soils used for cereal cropping in southern Queensland (31-67 kg N/ha.year). Average wheat grain yields on this site were approximately 0.8 t/ha.year, which would remove about 16 kg N/ha.year in the grain produce. Considering the fact that N additions through monoammonium phosphate fertiliser were about 2.5 kg N/ha.year, these studies corroborate earlier observations that soil N losses were primarily accounted for in the harvested grain (Dalal and Mayer 1986c). Soil N depletion in the buffel pasture 20 years after mulga clearing was 240 kg N/ha to a depth of 0.3 m (Table 4), or a loss of 12 kg N/ha.year. Large N losses from soil under pasture are not expected because N removal in animal produce (approximately 0.1 steer/ha.year or about 5 kg N/ha.year) from grazing would be much lower than that removed in grains from the cropping site (approximately 16 kg N/ha.year). It is also possible that N mineralised from soil organic N in the early spring (before the summer-grown pasture N uptake) may have been subjected to leaching losses in this soil.

Despite the significant differences in soil total N, soil [delta][sup.15]N values under mulga, pasture, and cropping were similar throughout the 0-1 m depths (4-5 [per thousand]) (Fig. 1), even after about 30% loss of soil N, indicating similar soil N sources for the 3 systems. Gathumbi et al. (2002) also reported little variation in [delta][sup.15]N values with depth for a number of [N.sub.2]-fixing species in Africa where [N.sub.2] fixation was the dominant N source, although soil [delta][sup.15]N values were almost double those measured in the present study. May and Attiwill (2003) used the [sup.15]N natural abundance technique to determine [N.sub.2] fixation by Acacia dealbata (silver wattle) and found that it contributed significantly to the total soil N pool of mountain ash (Eucalyptus regnans) forests in the Victorian central highlands, Australia.

[FIGURE 1 OMITTED]

The soil [delta][sup.15]N values in the present study are similar to that of soil N[O.sub.3.sup.-]-N (4.3 [per thousand]) under mulga as measured by Pate et al. (1998) and also similar to those values measured for the whole soil (3.2 [per thousand]) for A. dealbata in low-stocked mountain ash forests (May and Attiwill 2003). Black and Waring (1977) also found similar [delta][sup.15]N values for soil organic matter and soil-derived mineral N. As N[O.sub.3.sup.-]-N mineralised from soil organic N is the major source of N for both buffel pasture and cereal crops, which have no or very few legumes, large changes in [delta][sup.15]N values between the mulga and these areas were not expected in the whole soil N pool unless significant denitrification losses or rapid N transformation and recycling occurred (Shearer et al. 1974; Nadelhoffer and Fry 1988; Nadelhoffer and Fry 1994). Losses due to denitrification were assumed to be negligible in the current study because the sites rarely become anaerobic in this sandy soil due to the semi-arid environment. It is likely, however, that significant changes in the labile soil N pool occurred due to the potential for [N.sub.2] fixation by mulga but not by non-[N.sub.2] fixing pasture and crops.

The C/N ratio of the organic matter under pasture and cropping increased in the top 0-0.1 m depth only (Table 5). In the top 0-0.05 m depth, the C/N ratio increased from 11.6 under mulga to 14.0 under wheat cropping, whereas in the 0.05-0.1 m depth, the C/N ratio was 12.9 for soil under mulga and 14.9 for soil under pasture. Consequently, N losses exceeded the organic C losses when the soil under mulga was cleared and used for pasture or cereal production, indicating that the quality of soil organic matter declined after this land use change.

Light fraction N (a measure of labile N pool)

Eight fraction (density <1.6 Mg/[m.sup.3]) N decreased significantly (>60%) at all soil depths under both pasture and cropping following mulga clearing (Table 6). Although light fraction N comprised only 5-11% of the total soil N under mulga, and 2-6% under pasture and cropping, large decreases in the light fraction may adversely affect soil organic matter quality (Christensen 1992; Gregorich and Janzen 1996) and, hence, potential plant growth.

Cumulative amounts of light fraction N down to 1 m depth under mulga, pasture, and cropping were 258, 87, and 97 kg N/ha, respectively (Fig. 2). The light fraction C/N ratio generally increased with depth (Fig. 3) and it was more than twice that of the whole soil organic matter (Table 5). However, it remained much lower under mulga (20-30) than under pasture and cropping (25-50), thus reflecting the differences in the quality of soil organic matter in the labile pool under the different land uses, which was less apparent in the whole soil organic matter. The turnover time of the light fraction N (1/k) varied from 16.5 to 24.6 years under pasture and from 16.4 to 29.2 years under cropping, with no consistent trend with depth. Thus, light fraction N turned over much faster than the whole soil N.

[FIGURES 2-3 OMITTED]

Although [delta][sup.15]N values of the whole soil N were similar under all 3 land uses with a mean value of 4.8 [per thousand] (Fig. 1), those of the light fraction N were significantly lower (or approaching atmospheric values) under mulga than under pasture and cropping at a number of soil depth intervals (Fig. 4). Generally, light fraction [delta][sup.15]N values under mulga decreased with depth, from 2.4 [per thousand] in the 0-0.05 m depth to -0.3 [per thousand] in the 0.6-1.0 m soil depth (l.s.d. (P = 0.05) = 0.5 [per thousand]). This indicates that much of the light fraction N at depth under the mulga may be sourced from biological [N.sub.2] fixation, with much less N cycling than in the top layers. These trends with depth were inconsistent in soil under pasture and cropping, as the light fraction N in these soils is probably derived from both the original mulga vegetation and decomposing roots of pasture or crop plants.

[FIGURE 4 OMITTED]

The light fraction organic matter values of [[delta].sup.13]C (Dalal et al. 2005) and [delta][sup.15]N were closely associated in soil under wheat, only slightly associated in soil under pasture, but poorly associated in soil under mulga (Fig. 5). It is likely that most of the N uptake and removal by wheat was primarily derived from the N mineralised from the increasingly [sup.13]C-and [sup.15]N-enriched pool of soil organic matter, whereas in pasture soil N immobilisation is a significant process of N cycling due to the high C input by grass.

[FIGURE 5 OMITTED]

On average for the whole soil profile, light fraction [delta][sup.15]N values were 1.1 [per thousand], 1.9 [per thousand], and 2.1 [per thousand] (l.s.d. (P = 0.05) = 0.4 [per thousand]), under mulga, pasture and cropping. respectively, compared with a much higher [sup.15]N-enrichment of the whole soil N under all 3 systems (mean [delta][sup.15]N value, 4.8 [per thousand]), suggesting lesser plant N decomposition and transformation (Shearer et al. 1974; Nadelhoffer and Fry 1988, 1994) in the light fraction. Also, light fraction N under mulga was [sup.15]N-depleted compared with pasture and cropping, indicating greater [N.sub.2] fixation in soil under mulga. Mulga and buffel vegetation [delta][sup.15]N values (Table 2) were much closer to the light fraction [delta][sup.15]N values than those of the whole soil, suggesting that the light fraction is closely associated with or primarily derived from plant biomass (Dalal et al. 2005). Wheat [delta][sup.15]N values were significantly higher than the buffel [delta][sup.15]N values, possibly because increasingly [sup.15]N-enriched organic N is mineralised and taken up by wheat, whereas substantial mineralised N is immobilised in soil under pasture due to large C input from litter and roots.

Estimates of [N.sub.2] fixation by mulga

Analysis of mulga phyllodes, mulga twigs, and mulga fine roots from the study site gave [delta][sup.15]N values of -0.5 [per thousand], -0.8 [per thousand], and -0.5 [per thousand], respectively, compared with 8.9 [per thousand] and 9.3 [per thousand] for wheat shoots and wheat roots, respectively. The [delta][sup.15]N values for buffel pasture shoots and roots and ironbark leaves were intermediate (2.0 [per thousand], 1.6 [per thousand], 0.9 [per thousand], respectively) between mulga and wheat (Table 2). Schmidt and Stewart (2003) reported that all parts of [N.sub.2]-fixing species studied had negative [delta][sup.15]N values (except leaves of a monsoon Acacia species) and that these species had significantly higher leaf N contents than non-fixing species. The data in Table 2 support this theory, with all mulga parts (phyllodes, twigs, and fine roots) having negative [delta][sup.15]N values and high N contents compared with the buffel, ironbark, and wheat components, which had positive [delta][sup.15]N values and lower N contents than the mulga. [delta][sup.15]N values of mulga vegetation in this study are similar to that of an Acacia species (-0.4 [per thousand]) measured by Mordelet et al. (1996) in an Australian savanna and in the [N.sub.2]-fixing species in an Australian escarpment woodland (-0.2 [per thousand]) as reported by Schmidt and Stewart (2003), but are higher than those values (-0.66 to -1.13 [per thousand]) reported by May and Attiwill (2003) for A. dealbata.

Assuming ironbark (growing among the mulga forest) leaves and buffel tops as the reference plants (since buffel grass received no fertiliser N over the last 20 years after establishment following mulga clearing and most of the N in shoots was derived from root decomposition; Abbadie et al. 1992), the percentage of [N.sub.2] derived from the atmosphere (%Ndfa) by mulga can be calculated as follows:

(1) %Ndfa = [100 x ([delta][sup.15]N reference plant - [delta][sup.15]N mulga)]/ ([delta][sup.15]N reference plant - B)

where [delta][sup.15]N reference plant (ironbark) and buffel shoots and [delta][sup.15]N mulga phyllodes are 0.9 [per thousand], 2 [per thousand], and -0.5 [per thousand], respectively, and B is the [delta][sup.15]N of fixed [N.sub.2] for mulga (nodulated legume grown in N-free media, -1.1 [per thousand]; Lajtha and Marshall 1994; Yoneyama 1996). The B value used in this study is similar to the mean value (- 1.06 [per thousand]) of >35 datasets collated by Boddey et al. (2000), including the values given by Okito et al. (2004) and -1.3 [per thousand] used for Acacia dealbata by May and Attiwill (2003). Using Eqn 1 above and ironbark and buffel as the reference plants, it is estimated that 70% (using ironbark as the reference plant) to 80% (using buffel as the reference plant) of N in mulga phyllodes was derived from the atmosphere. Annual litter contained 1800 kg/ha (data not presented), and with an N content of 1.86% (Table 1), annual litter fall produced 33 kg N/ha, of which 23-26 kgN/ha (approximately 25 kg N/ha) is estimated to be the proportion of [N.sub.2] fixed annually, provided N mulga vegetation is at steady-state. Langkamp et al. (1979) estimated [N.sub.2] fixation (acetylene reduction technique) of only 12 kg N/ha.year for an Acacia species in northern Australia. May and Attiwill (2003) reported [N.sub.2] fixation of moderately stocked A. dealbata in an unburnt mountain ash forest to be 56 kg N/ha, but at high stocking rates this increased to 134 kg N/ha. However, further work is required to ascertain the sources and mechanisms of [N.sub.2] fixation by mulga (A. aneura) or other species, such as cyanobacteria associated with surface microbiota (Aranibar et al. 2003), which may also be associated with mulga. This work is needed because Beadle (1964) and Pate et al. (1998) found no evidence of [N.sub.2] fixation by mulga in the field. In addition, there is a need to measure the B value in mulga grown for a longer period, preferably under field conditions, although Ledgard and Peoples (1988) found that the B value can be established in glasshouse experiments and successfully used to estimate %Ndfa using the [sup.15]N natural abundance technique.

Mineralisable nitrogen

Mineralisation of soil organic N and turnover of small roots are recognised as the main pools of 'available' N to many land systems, with the exception of [N.sub.2]-fixing species where symbiotic [N.sub.2] fixation may be an important part of N cycling (Keeney 1980). Mineral N contents (mostly N[O.sub.3.sup.-]-N but also N[H.sub.4.sup.+]-N) were determined in the field in October 2003 and again in December 2003 after steel cores were left to incubate in the field for 2 months. Results are shown in Fig. 6. Soil N[O.sub.3.sup.-]-N under mulga was highest in both the October and December sampling periods compared with that for pasture and cropping (Fig. 6), with N[O.sub.3.sup.-]-N values of 44, 26, and 24 kg N/ha in the 1 m soil depth under mulga, pasture, and cropping, respectively, for the December sampling period. The latter sampling excluded mineralised N taken up by growing roots after the October sampling. Similar decreases in N[O.sub.3.sup.-]-N content and net N mineralisation rates were found following conversion of native forest to pasture in Ecuador (Rhoades and Coleman 1999), the Brazilian Amazon Basin (Neill et al. 1999), and Costa Rica (Montagnini and Sancho 1994). Montagnini and Sancho (1994) studied mineralisation rates of monoculture native plantations, an abandoned pasture, and a 20-year-old secondary forest. They concluded that daily soil nitrification rates were highest under [N.sub.2]-fixing leguminous species and that these values were comparable to the secondary forest. Mulga, as a leguminous tree species, has a high potential to fix atmospheric [N.sub.2], and a greater amount of nitrate N is expected compared with pasture and crops, yet further work is needed to establish whether [N.sub.2] is fixed by mulga or cyanobacterial crusts (Aranibar et al. 2003).

[FIGURE 6 OMITTED]

Neill et al. (1999) studied net N mineralisation rates of undisturbed tropical forest, as well as 4-, 10-, and 21-year-old pasture. They reported that rates of net N mineralisation and net nitrification decreased from forest to 21-year-old pasture. Gross N mineralisation rates were similar for forest, 4- and 10-year-old pasture, then declined in the 21-year-old pasture, indicating that when forests are converted to pasture, soil N turnover is maintained for a decade or so, then eventually slows in old pastures (Neill et al. 1999). The pasture area in this study was more than 20 years old, suggesting that the decreased nitrification rate under pasture may be a combination of decreased labile N under pasture and [N.sub.2] fixation inputs under mulga. Nitrate-N values determined in situ display a similar trend to that of light fraction N in these soils under mulga, pasture, and cropping; therefore, it can be inferred that after mulga clearing and the establishment of pasture and cropping, there would be lower N supply to plants. This would result in lower soil organic C and N concentrations and lower soil fertility and, hence, may affect plant productivity.

Conclusion

Soil N losses, estimated to be 12 and 17 kg N/ha/year under pasture and cropping, have depleted the soil N resource by 240 and 340 kg/ha, respectively, after mulga clearing. More than 60% of the light fraction N, a measure of labile N, had disappeared from the top 1 m soil depth. Therefore, there is a concern for the sustainable use of the Mulga Lands cleared for pasture and cropping, with a continuing decline in soil organic matter density, soil organic matter quality, mineralisable N, and, hence, soil fertility and plant growth. Consequently, the removal of mulga forest over a 20-year period in Queensland for pasture and cropping may not only have resulted in soil C losses and an increased greenhouse effect (Dalal et al. 2005) but also lowered the fertility of the cleared Mulga Lands. Understanding of C and N dynamics (and other nutrients) and their management in these soils is essential for land sustainable use and for devising effective greenhouse gas mitigation practices for Mulga Lands.
Table 1. Soil characteristics (mulga site)

Soil depth Soil pH Organic C Sand Silt Clay
(m) (1 : 5 [H.sub.2]O) (%) (%) (%) (%)

 0-0.05 6.1 0.96 82 6 12
 0-0.1 6.0 0.69 81 6 13
 0.1-0.2 6.0 0.51 79 5 16
 0.2-0.3 6.1 0.44 79 5 16
 0.3-0.6 6.0 0.27 78 4 18
 0.6-1.0 6.2 0.17 75 3 22

Table 2. [[delta].sup.15]N values of mulga phyllodes, mulga twigs
(5-20 mm), mulga fine roots (<5 mm), buffel shoots, buffel roots,
and wheat stubble and roots at harvest

Plant parts [delta][sup.15]N value (%o) N content (%)

Mulga phyllodes (5) -0.49 [+ or -] 1.30 1.86
Mulga twigs (5) -0.78 [+ or -] 1.01 1.18
Mulga fine roots (3) -0.54 [+ or -] 0.46 1.66
Ironbark leaves (3) 0.87 [+ or -] 0.15 1.20
Buffel shoots (3) 2.02 [+ or -] 0.09 0.63
Buffel roots (3) 1.57 [+ or -] 0.09 0.51
Wheat stubble (1) 8.9 0.80
Wheat roots (1) 9.3 0.94

Table 3. Total soil nitrogen concentration (%) under mulga,
pasture, and cropping

Soil depth Mulga Pasture Cropping l.s.d.
(m) (P = 0.05)

 0-0.05 0.074 0.053 0.044 0.010
0.05-0.1 0.050 0.043 0.036 0.009
 0.1-0.2 0.037 0.036 0.035 n.s.
 0.2-0.3 0.033 0.031 0.027 0.004
 0.3-0.6 0.022 0.024 0.024 n.s.
 0.6-1.0 0.017 0.018 0.017 n.s.

n.s., Not significant.

Table 4. Cumulative soil profile total nitrogen (kg/ha) under
mulga, pasture, and cropping

Soil depth Mulga Pasture Cropping l.s.d.
(m) (P = 0.05)

0-0.05 519 371 307 50
0-0.1 870 675 559 85
0-0.2 1392 1186 1055 151
0-0.3 1879 1641 1452 194
0-0.6 2834 2668 2467 309
0-1.0 3812 3690 3470 331

Table 5. Soil C/N ratio under mulga, pasture, and cropping

Soil depth Mulga Pasture Cropping l.s.d.
(m) (P = 0.05)

 0-0.05 11.6 12.6 14.0 1.8
0.05-0.1 12.9 14.9 13.7 1.8
 0.1-0.2 13.6 14.4 12.8 n.s.
 0.2-0.3 12.8 12.4 13.9 n.s.
 0.3-0.6 11.7 11.2 10.6 n.s.
 0.6-1.0 9.9 9.8 10.2 n.s.

n.s., Not significant.

Table 6. Light fraction soil nitrogen (kg/ha) under mulga, pasture
and cropping

Soil depth Mulga Pasture Cropping l.s.d.
(m) (P = 0.05)

 0-0.05 58.9 17.5 19.6 14.6
0.05-0.1 28.0 12.4 14.1 5.2
 0.1-0.2 39.6 13.7 14.9 10.7
 0.2-0.3 32.2 9.7 11.1 6.6
 0.3-0.6 49.4 17.1 18.6 13.6
 0.6-1.0 50.6 16.2 18.9 25.2
Total (0-1.0 m) 258.7 86.6 97.2 57.4


Acknowledgments

We thank Ian Hill of 'Mulga View', St George, for his permission to access the site, Bruce Scriven for providing the past history of the site, and Rory Whitehead, Christine McCallum and Analytical Services staff for their technical assistance, Kamal Sangha for statistical analysis, Rene Diocares for [[delta].sup.15]N analysis of ironbark leaves, and John Raison, Roger Gifford, and Jeff Baldock for their suggestions.

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Manuscript received 16 June 2004, accepted 10 November 2004

R. C. Dalal (A,B,D), B. P. Harms (A,B), E. Krull (A,C), W. J. Wang (A,B), and N. J. Mathers (A,B)

(A) CRC for Greenhouse Accounting.

(B) Department of Natural Resources and Mines, Indooroopilly, Qld 4068, Australia.

(C) CSIRO Land and Water, Glen Osmond, SA 5064, Australia.

(D) Corresponding author. Email: Ram.Dalal@nrm.qld.gov.au
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Author:Dalal, R.C.; Harms, B.P.; Krull, E.; Wang, W.J.; Mathers, N.J.
Publication:Australian Journal of Soil Research
Geographic Code:8AUST
Date:Mar 1, 2005
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