Empirical model for mineralisation of manure nitrogen in soil

A simple empirical model was developed for estimation of net mineralisation of pig and cattle slurry nitrogen (N) in arable soils under cool and moist climate conditions during the initial 5 years after spring application. The model is based on a Danish 3-year field experiment with measurements of N uptake in spring barley and ryegrass catch crops, supplemented with data from the literature on the temporal release of organic residues in soil. The model estimates a faster mineralisation rate for organic N in pig slurry compared with cattle slurry, and the description includes an initial N immobilisation phase for both manure types. The model estimates a cumulated net mineralisation of 71% and 51% of organic N in pig and cattle slurry respectively after 5 years. These estimates are in accordance with some other mineralisation studies and studies of the effects of manure residual N in other North European countries.

Fate of biuret 15N and its effect on net mineralisation of native soil N in forest soils

Biuret (C2H5N3O2) priming effect on mineralisation of native soil N has not been precisely quantified in previous studies, although it is a potential microbial activity regulator and slow-release N fertiliser. Following application of biuret at concentrations of 0 (B0) and 100 (B100) mg/kg (oven-dried) soil, we measured the dynamics of biuret-derived 15N in soil N pools, soil C mineralisation, and microbial biomass C in a sandy loam and a silt loam during a 112-day-long incubation to investigate the fate of biuret 15N and its effect on net mineralisation of native soil N. Biuret was decomposed faster in the sandy loam soil than the silt loam soil. In the sandy loam soil, the stabilised N pool was a strong sink for the biuret-derived 15N and accumulated about half of the applied 15N at the end of incubation. In the silt loam soil, 68% of the 15N applied was recovered in the NO3−-N pool and the stabilised N pool accumulated only about 25% of the applied 15N at the end of incubation. Biuret addition increased the turnover rate constant of soil organic matter and caused a real priming effect on net mineralisation of native soil N in both soils. The additional mineralisation of native soil N was 20.1 mg/kg (equivalent to 27.3 kg N/ha) in the sandy loam soil and 20.5 mg/kg (equivalent to 57.3 kg N/ha) in the silt loam soil. Biuret priming effect was related to the acceleration of soil organic matter decomposition by increased microbial activity at an early stage and the death/decay of microbes at a later stage of incubation. The native soil N released through the priming effect was partially from soil non-biomass organic matter and partially from soil microbial biomass.

Nitrogen mineralisation in relation to previous crops and pastures

Most of the nitrogen (N) used by Australian crops is mineralised from the residues of previous crops and pastures. Net N mineralisation was studied in 2 field experiments in southern NSW, one comparing different residue-management and tillage systems during continuous cropping and the other comparing residues of annual and perennial pastures in a pasture–crop system. After 14 years of continuous cropping, soil total N concentration had decreased by 50%. Neither stubble retention nor direct drilling affected potential N mineralisation or the decrease in total N. However, soil mineral N in the field was greater after direct drilling than cultivation and greater after stubble retention than stubble burning. There were 2 reasons for the discrepancy. One was because retained stubble conserved soil water, leading to periods of increased mineralisation. The other was that direct drilling and stubble retention reduced growth and N uptake by crops. In contrast to the similar rates of potential mineralisation under different tillage and stubble systems, there were significant differences following different pasture species. In a 5-year study of a pasture–crop system we measured net mineralisation following annual pasture based on subterranean clover and perennial pasture based on lucerne and/or the grasses phalaris and cocksfoot. Mineralisation generally decreased with number of years after pasture removal. Previous lucerne pastures led to slow net mineralisation in the first year after removal, apparently because of immobilisation by high C : N residues. Mineralisation in soil containing perennial grass residues was the highest measured. This high rate may be due to redistribution of N to the topsoil by roots of perennial grasses. The comparison of continuous crop and pasture–crop systems showed that the decline in soil N supply was not prevented by direct drilling and stubble conservation, but N mineralisation was increased by pastures, particularly those containing perennial grasses.

Changes in free living soil nematode and micro-arthropod communities under a canola - wheat - lupin rotation in Western Australia

Diversification of the crops used in wheat production systems provides alternative sources of income and can interrupt wheat pathogen lifecycles. Two important alternative crops in Western Australia are canola and lupins, which may both improve growth of following wheat. Improved growth of wheat following canola may be the consequence of biofumigation or increased root penetration by the wheat. Available nitrogen may be increased following lupins. We examined free-living soil fauna in a canola–wheat–lupin rotation near Moora, Western Australia, to determine the effects of these crops on the soil fauna. Each crop in the rotation was sampled in June, August, and October 1998. Nematodes were sorted into functional groups and arthropods were sorted to order level. Prostigmatid mites were the dominant arthropod group and they were sorted to morphospecies. An active and abundant faunal community was present under all crops, demonstrating that the canola variety in this study, Pinnacle TT, did not eliminate the free-living fauna. The structure of the mite communities changed throughout the year and the changes were different under the 3 crops. The soil arthropod communities were distinctly different under lupins compared with the other crops at the end of the growing season in 2 ways. First, 5 times more animals were present under the lupins than under wheat or canola, primarily due to an increase in the numbers of a tydeid and a tarsonemid mite species. Second, the tarsonemid species was always the second most abundant species under lupins but was infrequently the second ranked species under the other 2 crops. The soil arthropod communities were also different at the start of the growing season when the prostigmatid community under canola was dominated by a rhagidiid species, whilst under lupins and wheat a caligonellid and eupodid species dominated. The canola followed a lupin crop and therefore the difference in June may be attributed to the preceding lupins. Mite data from the lupin plots were consistent with a previously described succession from another environment. We hypothesise that if net nutrient mineralisation rates are greatest at the start of a succession then net mineralisation rates under lupins may be rapid at the end of the lupin crop and slow when the next crop is planted in the remaining lupin stubble. The difference between lupins and canola in their mite communities would then imply that net mineralisation rates are a factor creating differences between the effects of break crops on the following wheat crop.

Nitrogen budget of wheat growing on a Riverine clay soil

The fate of nitrogen in wheat grown on a Mesotrophic, Red Kandosol near Wagga Wagga was studied in the 1993 growing season, which had above-average rainfall: 417 mm (31 May–30 November 1993) compared with an average (June–November) of 289 mm. Nitrogen supply (fertiliser and mineralisation) was partitioned between crop uptake, gaseous and leaching losses, and residual mineral N in the soil profile. The study plots were 2 adjacent 5-ha areas. At stem elongation (Zadock’s decimal code 31), one area was topdressed with urea at 14 g N/m2 (fertilised crop). The total N supply to the fertilised crop was 29 g N/m2—8 g N/m2 of mineral N in the soil at sowing, net mineralisation of 5.3 g N/m2, and fertiliser inputs of 1.7 and 14 g N/m2. The corresponding value for the non-fertilised crop was 15 g N/m2. The urea application produced a 50% increase in above-ground biomass (1521 and 1008 g/m2 dry matter at harvest) and a 1.8-fold increase in grain yield (692 and 384 g/m2). The proportion of the total N supply recovered in the crops was similar (55% and 60% for the non-fertilised and fertilised treatments, respectively). Leaching losses were low (0.4 and 0.5 g N/m2), even though ≈100 mm drained beyond the root-zone (equivalent to 24% of the seasonal rainfall). The periods of saturated soil required to generate drainage also caused denitrification losses of 1.7 and 3.4 g N/m2 for the non-fertilised and fertilised treatments, respectively. Increased net mineralisation and reduced crop N uptake that began a month prior to anthesis were responsible for the substantial amounts of mineral N remaining in the soil after harvest (4.7 and 4.3 g N/m2, respectively). The low NO3 leaching loss associated with high drainage was explained by displacement flow mechanics operating in soil that has a high water retention capacity, which is confirmed by Br and 15N tracer analysis. The N balance was closed for the non-fertilised crop, but a discrepancy of 2.8 g N/m2 remains for the fertilised crop. The uncertainty of ≈10% of the fertilised treatment may possibly be due to ammonia volatilisation following topdressing with urea.

Turnover of nitrogen from components of lupin stubble to wheat in sandy soil

The rate of decomposition of 15N-labelled lupin (Lupinus angustifolius) stubble and the use of mineralised 15N by wheat were determined in field experiments on a deep loamy sand previously cropped to lupin. In one experiment, leaf, stem, and pod (pod-valve) components were applied separately to mini-plots that were either left unplanted or subsequently planted to wheat. In the second experiment, leaf and stem components, each of either low or high N concentration, were applied separately to mini-plots which were subsequently planted to wheat. Soil was recovered in layers to a maximum depth of 1 m and subsequently analysed for 15N in NH + 4 , NO-3 , and total N. The net mineralisation of stubble 15N was estimated from the decrease in soil organic 15N (total 15N – inorganic 15N), and the uptake of 15N by wheat was measured periodically. All treatments were characterised by the high retention of lupin stubble 15N in the soil organic matter. Between 9 and 34% of stem and pod 15N, and 19–49% of leaf 15N, was mineralised within a 10-month period. From these data the annual net mineralisation of a typical lupin stubble was estimated at 25–42 kg N/ha, an N benefit similar to that estimated from agronomic trials. Wheat uptake of lupin-stubble 15N ranged from 9 to 27%. Of the stubble components, only the leaf contained sufficient quantities of mineralisable N to be an important source of N for wheat. At wheat maturity in the first experiment, losses of stubble 15N ranged from 13% (leaf) to 7% (stem). In the second experiment, losses of 15N were only observed from the high N treatments (leaf 8%, stem 15·5%). Stubble component chemistry appeared to affect net mineralisation and plant uptake differently. Across both experiments, annual net mineralisation best correlated (R = 0·69) with the N concentration of the stubble components. Wheat N uptake was strongly positively correlated with polysaccharide content (R = 0·89) but negatively correlated with lignin content (R = – 0·79). Although large quantities (58 and 98 kg N/ha) of soil-derived inorganic N were found in the root-zone (–1·0 m) of wheat sown after lupins, and attributed to the decomposition of lupin root systems and surface residues prior to the establishment of each experiment, it is concluded that the short-term decomposition of lupin stubble 15N results in a modest release of inorganic N. Consequently, the primary value of lupin stubble in the N economy of lupin : cereal rotations is to replenish the soil organic N reserve.

The source of mineral nitrogen for cereals in south-eastern Australia

The relative importance of soil mineral nitrogen (N) available at the time of sowing ormineralised during the growing season was investigated for 6 crops of dryland wheat. The soil mineral N in the root-zone was sampled at sowing and maturity and the rate of net mineralisation in the top 10 cm was estimated by sequential sampling throughout the growing season, using an in situ method. Mineralisation during crop growth was modelled in relation to total soil N, ambient temperature, andsoil water content. Mineral N accumulated before sowing varied by a factor of 3 between the sites (from 67 to 195 kgN/ha), while the net mineralisation during crop growth varied by a factor of 2 (from 43 to 99 kgN/ha). The model indicated that 0·092% of total N was mineralised per day when temperature and water were not limiting, with rates decreasing for lower temperatures and soil water contents. When tested with independent data, the model predicted the mineralisation rate of soil growing continuous wheat crops but underestimated mineralisation of soil in a clover-wheat rotation. For crops yielding <3 t/ha, the supply of N was mostly from mineralisation during crop growth and the contribution from mineral N accumulated before sowing was relatively small. For crops yielding >4 t/ha, thesupply of N was mostly from N present in the soil at the time of sowing. The implication is that for crops to achieve their water-limited yield, they must be supplied with an amount of N greater than can be expected from mineralisation during the growing season, either from fertiliser or from mineral N accumulated earlier.

Estimates of lupin below-ground biomass nitrogen, dry matter, and nitrogen turnover to wheat

The amount of lupin below-ground biomass (BGB), BGB nitrogen (N) content, and utilization of BGB-N by subsequent wheat was estimated from lupins grown in soil columns. Lupin plants were enriched in situ with 15N-labelled urea through a cotton wick inserted through the stem. Of the applied 15N. 92% was recovered in the lupin plant-soil system at maturity: 87% of this 15N was in lupin aboveground biomass and 13% in the soil columns. Total mature lupin dry matter (DM) approximated 11 t/ha, with 3.0 t/ha (27%) of this DM below ground. Total mature lupin N approximated 321 kg/ha, of which 91 kg/ha (28%) resided below ground. In terms of N and DM, BGB was the largest lupin residue component even though only 35% of this was recoverable as root material. About 13% of the BGB-N was in inorganic form at maturity. The net mineralisation of lupin BGB-N after 2 consecutive years of wheat growth was 27%. and wheat assimilated about 74% of this N (i.e. 20% of BGB-N), with equal quantities assimilated in each year. The contribution of lupin BGB-N to the N in wheat tops ranged from 40% for soil columns receiving no fertiliser N to 15-20% for soil columns fertilised with 30 kg N/ha. The net mineralisation of BGB-N and the assimilation of BGB-N by wheat were unaffected by the application of fertiliser N.