Dissolved organic nitrogen contributes significantly to leaching from furrow-irrigated cotton–wheat–maize rotations

Soil Research ◽  
2017 ◽  
Vol 55 (1) ◽  
pp. 70 ◽  
Author(s):  
B. C. T. Macdonald ◽  
A. J. Ringrose-Voase ◽  
A. J. Nadelko ◽  
M. Farrell ◽  
S. Tuomi ◽  
...  
Keyword(s):  
Soil Surface ◽  
Significant Loss ◽  
Organic N ◽  
Irrigated Agriculture ◽  
Cotton Production ◽  
N Loss ◽  
Deep Drainage ◽  
Mineral N ◽  
Undisturbed Soil ◽  
Total N

Leaching of nitrogen (N) in intensive irrigated agriculture can be a significant loss pathway. Though many studies have focussed on losses of mineral N, and in particular nitrate, dissolved organic N (DON) has received less coverage. In the present study, over a 5-year period (2008–2013), 740kgNha–1 fertiliser was applied to an irrigated cotton–wheat–maize rotation on a cracking clay (grey Vertosol). Deep drainage from the undisturbed soil profile at the site was measured at 2.1m below the soil surface using a variable tension lysimeter. In total, 108mm of drainage occurred during the 5 years and the majority of the drainage and the irrigations occurred during the cotton seasons. The majority of the N loss occurred during the first 3–4 irrigations and neither the N loss nor its composition were affected by the product or timing of the fertiliser application. The N in the drainage was composed of 12.8kgNOx-Nha–1, 8.7 DON-N and 0.1 NH4+-Nkgha–1, which shows that DON is an important component (40%) of the deep drainage N from irrigated Vertosol cotton production systems. Overall the total N flux lost via deep drainage represents 3% of the applied N fertiliser.

Nitrogen leaching from soil lysimeters irrigated with dairy shed effluent and having managed drainage

Soil Research ◽  
2001 ◽  
Vol 39 (2) ◽  
pp. 385 ◽  
Author(s):  
P. L. Singleton ◽  
C. D. A McLay ◽  
G. F. Barkle
Keyword(s):  
Water Table ◽  
Soil Surface ◽  
World Health ◽  
Organic N ◽  
Similar Proportion ◽  
N Leaching ◽  
Undisturbed Soil ◽  
Total N ◽  
Leaching Losses ◽  
N Form

The leaching of nitrogen (N) from agricultural soils is undesirable for environmental and health reasons. We investigated the effects of adding dairy shed effluent (DSE), irrigated on a weekly basis during the milking season, on the amounts and forms of N leached from large undisturbed soil monolith lysimeters of a Gley Soil over a period of 2 years. Drainage was managed using a weir that maintained the water table at 3 depths: 25 (high), 50 (medium), or 75 (low) cm below the soil surface. The low water table treatment represented the usual situation for the soil when drained. If undrained, it would be usual during wet periods in the field for a perched water table to form on the slowly permeable horizon at 75 cm depth. The total amount of N irrigated onto the lysimeters in the first milking season was equivalent to a total of 511 kg N/ha.year, and up to 33.3 kg N/ha.year leached from the soil. The losses from lysimeters receiving effluent were nearly double those from lysimeters receiving an equivalent amount of water only, when the high and medium water tables were imposed. Adding effluent caused only a small increase (7 kg N/ha) in total N leached in the low drainage treatment. In the second milking season, the effluent-N loading was increased to 1518 kg N/ha.year and the pasture was managed to simulate a ‘cut and carry’ land treatment system. Under these conditions, up to 131.4 kg N/ha.year leached from the soil, which was nearly 100 kg N/ha more than lysimeters receiving only water. The total N leaching losses represented a similar proportion of added N (7% and 9%) for years 1 and 2, respectively. Most of the leached N (80—90%) was in organic N form. The managed drainage treatment in which the water table was nearest the soil surface resulted in less N being leached in the nitrate-N (NO 3 -N) form (<2.5 kg N/ha.year) than the other drainage treatments (6—12 kg N/ha.year); however, it did result in the greatest amount of organic and total N leached (33 and 131 kg N/ha for Year 1 and 2, respectively). The smaller amount of NO 3 -N leached from the high water table treatment is attributed to enhanced denitrification, and the greater amount of organic N is attributed to preferential flow. Although NO 3 -N concentrations in leachate generally remained below World Health Organisation (WHO) standards in all treatments, the large amount of N leached in organic form would suggest that inorganic N should not be the only form of N considered when measuring N leaching losses.

scholarly journals Effects of two wood-based biochars on the fate of added fertilizer nitrogen—a 15N tracing study

2021 ◽  
Author(s):  
Subin Kalu ◽  
Gboyega Nathaniel Oyekoya ◽  
Per Ambus ◽  
Priit Tammeorg ◽  
Asko Simojoki ◽  
...  
Keyword(s):  
Plant Uptake ◽  
Plant Biomass ◽  
Application Rate ◽  
Control Treatment ◽  
Organic N ◽  
N Leaching ◽  
Soil N ◽  
Mineral N ◽  
Total N ◽  
Application Rates

AbstractA 15N tracing pot experiment was conducted using two types of wood-based biochars: a regular biochar and a Kon-Tiki-produced nutrient-enriched biochar, at two application rates (1% and 5% (w/w)), in addition to a fertilizer only and a control treatment. Ryegrass was sown in pots, all of which except controls received 15N-labelled fertilizer as either 15NH4NO3 or NH415NO3. We quantified the effect of biochar application on soil N2O emissions, as well as the fate of fertilizer-derived ammonium (NH4+) and nitrate (NO3−) in terms of their leaching from the soil, uptake into plant biomass, and recovery in the soil. We found that application of biochars reduced soil mineral N leaching and N2O emissions. Similarly, the higher biochar application rate of 5% significantly increased aboveground ryegrass biomass yield. However, no differences in N2O emissions and ryegrass biomass yields were observed between regular and nutrient-enriched biochar treatments, although mineral N leaching tended to be lower in the nutrient-enriched biochar treatment than in the regular biochar treatment. The 15N analysis revealed that biochar application increased the plant uptake of added nitrate, but reduced the plant uptake of added ammonium compared to the fertilizer only treatment. Thus, the uptake of total N derived from added NH4NO3 fertilizer was not affected by the biochar addition, and cannot explain the increase in plant biomass in biochar treatments. Instead, the increased plant biomass at the higher biochar application rate was attributed to the enhanced uptake of N derived from soil. This suggests that the interactions between biochar and native soil organic N may be important determinants of the availability of soil N to plant growth.

Characteristics of Non-Point Source N Export via Surface Runoff from Sloping Croplands in Northeast China

2011 ◽  
Vol 347-353 ◽  
pp. 2302-2307 ◽  
Author(s):  
Hong Xiang Wang ◽  
Yi Shi ◽  
Jian Ma ◽  
Cai Yan Lu ◽  
Xin Chen
Keyword(s):  
Point Source ◽  
Surface Runoff ◽  
Rainfall Intensity ◽  
Inorganic Nitrogen ◽  
Rainfall Amount ◽  
Organic N ◽  
Dissolved Inorganic Nitrogen ◽  
Slope Gradient ◽  
N Loss ◽  
Total N

A field experiment was conducted to study the characteristics of non-point source nitrogen (N) in the surface runoff from sloping croplands and the influences of rainfall and cropland slope gradient. The results showed that dissolved total N (DTN) was the major form of N in the runoff, and the proportion occupied by dissolved inorganic nitrogen (DIN) ranged from 45% to 85%. The level of NH4+-N was generally higher than the level of NO3--N, and averaged at 2.50 mg·L-1and 1.07 mg·L-1respectively. DIN was positively correlated with DTN (R2=0.962). Dissolved organic N (DON) presented a moderate seasonal change and averaged at 1.40 mg·L-1. Rainfall amount and rainfall intensity significantly affected the components of DTN in the runoff. With the increase of rainfall amount and rainfall intensity, the concentrations of DTN, NH4+-N and NO3--N presented a decreased trend, while the concentration of DON showed an increased trend. N loss went up with an increase in the gradient of sloping cropland, and was less when the duration was longer from the time of N fertilization.fertilization.

The influence of the soil matrix on nitrogen mineralisation and nitrification. I. Spatial variation and a hierarchy of soil properties

Soil Research ◽  
1998 ◽  
Vol 36 (3) ◽  
pp. 429 ◽  
Author(s):  
D. T. Strong ◽  
P. W. G. Sale ◽  
K. R. Helyar
Keyword(s):  
Soil Properties ◽  
Water Content ◽  
Organic N ◽  
Small Field ◽  
N Mineralisation ◽  
Mineral N ◽  
Conceptual Tool ◽  
Soil Matrix ◽  
Total N ◽  
Soil Variables

Natural heterogeneity of soil properties was used to explore their influence on nitrogen (N) mineralisation and nitrification in undisturbed small soil volumes (soil cells; c. 1 · 7 cm3 ) sampled from a small field plot (2 m by 3 m). Soil cells (840) were randomly ascribed to 1 of 6 treatments in which soils were retained continuously moist (M10 and M30 treatments) and amended with organic N from clover (Cl10 and Cl30 treatments), dried and rewetted (DW10), or treated with urea (Ur10) (subscripts indicate soil incubation at matric potential - 10 or - 30 kPa). After 20 days of incubation at 24C, each soil cell was analysed for NO-3 -N, NH + 4 -N, pH, bulk density (BD), volumetric water content (θv), water content at - 490 kPa (θv490), and pH buffer capacity (pHBC). On 25 soil cells from each treatment, % clay, % silt, % sand, total N (% N), organic carbon (% C), and 7 cations and anions were also determined. Net N mineralisation and net nitrification occurred in all treatments, and the total mineral N at the end of the incubation was 497, 81, 73, 31, 27, and 31 µg N/g in the Ur10 Cl10, Cl30, M10, M30, and DW10 treatments, respectively. Net N mineralisation in the M30 treatment was 84% of that in the M10 treatment, and net N mineralisation in the Cl30 treatment was 86% of that in the Cl10 treatment. Fluctuations in soil pH varied markedly between treatments and over time, and it was apparent that alkaline processes were occurring in all soil cells. The heterogeneity between soil samples was substantial for all of the soil variables. Soil variables were classified in a hierarchy from the least to the most fundamental based on their stability through time. This ranking provides a conceptual tool for understanding interrelationships between soil properties and for interpreting results of regression analyses. The sampling approach adopted in this study was designed to harness the natural heterogeneity of soil properties in the small field site while keeping other properties and environmental factors, that usually vary over larger distances, constant. Both the extent of heterogeneity of soil properties and the nature of their correlations with NO-3 -N suggested that this technique would be useful in the exploration of how soil properties influence N mineralisation and nitrification.

NO3- uptake and C exudation – do plant roots stimulate or inhibit denitrification?

2020 ◽  
Author(s):  
Pauline Sophie Rummel ◽  
Reinhard Well ◽  
Birgit Pfeiffer ◽  
Klaus Dittert ◽  
Sebastian Floßmann ◽  
...  
Keyword(s):  
Soil Moisture ◽  
N Uptake ◽  
Root Exudation ◽  
Soil Surface ◽  
N Fertilization ◽  
Plant N Uptake ◽  
Mineral N ◽  
Double Labeling ◽  
Total N ◽  
Organic C

&lt;p&gt;Growing plants affect soil moisture, mineral N and organic C (C&lt;sub&gt;org&lt;/sub&gt;) availability in soil and may thus play an important role in regulating denitrification. The availability of the main substrates for denitrification (C&lt;sub&gt;org&lt;/sub&gt; and NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) is controlled by root activity and higher denitrification activity in rhizosphere soils has been reported. We hypothesized that (I) plant N uptake governs NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; availability for denitrification leading to increased N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; emissions, when plant N uptake is low due to smaller root system or root senescence. (II) Denitrification is stimulated by higher C&lt;sub&gt;org&lt;/sub&gt; availability from root exudation or decaying roots increasing total gaseous N emissions while decreasing their N&lt;sub&gt;2&lt;/sub&gt;O/(N&lt;sub&gt;2&lt;/sub&gt;O+N&lt;sub&gt;2&lt;/sub&gt;) ratios.&lt;/p&gt;&lt;p&gt;We tested these assumptions in a double labeling pot experiment with maize (Zea mays L.) grown under three N fertilization levels S / M / L (no / moderate / high N fertilization) and with cup plant (Silphium perfoliatum L., moderate N fertilization). After 6 weeks, all plants were labeled with 0.1 g N kg&lt;sup&gt;-1&lt;/sup&gt; (Ca(&lt;sup&gt;15&lt;/sup&gt;NO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;, 60 at%), and the &lt;sup&gt;15&lt;/sup&gt;N tracer method was applied to estimate plant N uptake, N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; emissions. To link denitrification with available C in the rhizosphere, &lt;sup&gt;13&lt;/sup&gt;CO&lt;sub&gt;2&lt;/sub&gt; pulse labeling (5 g Na&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;13&lt;/sup&gt;CO&lt;sub&gt;3&lt;/sub&gt;, 99 at%) was used to trace C translocation from shoots to roots and its release by roots into the soil. CO&lt;sub&gt;2&lt;/sub&gt; evolving from soil was trapped in NaOH for &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C analyses, and gas samples were taken for analysis of N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; from the headspace above the soil surface every 12 h.&lt;/p&gt;&lt;p&gt;Although pots were irrigated, changing soil moisture through differences in plant water uptake was the main factor controlling daily N&lt;sub&gt;2&lt;/sub&gt;O+N&lt;sub&gt;2&lt;/sub&gt; fluxes, cumulative N emissions, and N&lt;sub&gt;2&lt;/sub&gt;O production pathways. In addition, total N&lt;sub&gt;2&lt;/sub&gt;O+N&lt;sub&gt;2&lt;/sub&gt; emissions were negatively correlated with plant N uptake and positively with soil N concentrations. Recently assimilated C released by roots (&lt;sup&gt;13&lt;/sup&gt;C) was positively correlated with root dry matter, but we could not detect any relationship with cumulative N emissions. We anticipate that higher C&lt;sub&gt;org&lt;/sub&gt; availability in pots with large root systems did not lead to higher denitrification rates as NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; was limited due to plant uptake. In conclusion, plant growth controlled water and NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; uptake and, subsequently, formation of anaerobic hotspots for denitrification.&lt;/p&gt;

N2O fluxes in soils of contrasting textures fertilized with liquid and solid dairy cattle manures

2008 ◽  
Vol 88 (2) ◽  
pp. 175-187 ◽  
Author(s):  
Philippe Rochette ◽  
Denis A Angers ◽  
Martin H Chantigny ◽  
Bernard Gagnon ◽  
Normand Bertrand
Keyword(s):  
Dairy Cattle ◽  
Clay Soil ◽  
Soil Surface ◽  
Soil N ◽  
Soil Mineral N ◽  
Soil Mineral ◽  
Mineral N ◽  
Total N ◽  
Silage Maize ◽  
The Impact

Manure is known to increase soil N2O emissions by stimulating nitrification and denitrification processes. Our objective was to compare soil-surface N2O emissions following the application of liquid and solid dairy cattle manures to a loamy and a clay soil cropped to silage maize. Manures were applied in 2 consecutive years at rates equivalent to 150 kg total N ha-1 and compared with a control treatment receiving an equivalent rate of synthetic N. Soil-surface N2O fluxes, soil temperature, and soil water, nitrate and ammonium contents were monitored weekly in manured and control plots. From 60 to 90% of seasonal N2O emissions occurred during the first 40 d following manure and synthetic fertilizer applications, indicating that outside that period one or several factors limited N2O emissions. The period of higher emissions following manure and fertilizer application corresponded with the period when soil mineral N contents were highest (up to 17 g NO3−-N m-2) and water-filled pore space (WFPS) was greater than 0.5 m3 m-3. The absence of significant N2O fluxes later in the growing season despite high WFPS levels indicated that the stimulating effect of organic and synthetic N additions on soil N2O production was relatively short-lived. Fertilization of silage maize with dairy cattle manure resulted in greater or equal N2O emissions than with synthetic N. This was observed despite lower overall soil mineral N contents in the manured plots, indicating that other factors affected by manure, possibly additional C substrates and enhanced soil respiration, resulted in greater denitrification and N2O production. Silage maize yields in the manured soils were lower than those receiving synthetic N, indicating that the N2O emissions per kilogram of harvested biomass were greater for manures than for synthetic N. Our results also suggest that the main source of N2O was nitrification in the loam and denitrification in the clay soil. There was no clear difference in N2O emissions between liquid and solid manures. The variable effects of liquid and solid manure addition reported in the literature on soil N2O emissions likely result from the variable composition of the manures themselves as well as from interactions with other factors such as soil environment and farming practices. A better characterization of the availability of manure C and N is required to assess the impact of manure application on soil N2O emissions under field conditions. Key words: Greenhouse gases, N2O, maize, manure

scholarly journals Use of Digestate as Organic Amendment and Source of Nitrogen to Vegetable Crops

2021 ◽  
Vol 12 (1) ◽  
pp. 248
Author(s):  
Carmo Horta ◽  
João Paulo Carneiro
Keyword(s):  
Organic Amendment ◽  
Application Rate ◽  
Green Energy ◽  
Organic N ◽  
N Availability ◽  
Vegetable Crops ◽  
Mineral N ◽  
Total N ◽  
Ni Addition ◽  
Stable Organic Matter

Anaerobic digestion is a valuable process to use livestock effluents to produce green energy and a by-product called digestate with fertilising value. This work aimed at evaluating the fertilising value of the solid fraction (SF) of a digestate as an organic amendment and as a source of nitrogen to crops replacing mineral N. A field experiment was done with two consecutive vegetable crops. The treatments were: a control without fertilisation; Ni85 mineral fertilisation with 85 kg ha−1 of mineral N; fertiliser with digestate at an increasing nitrogen application rate (kg N ha−1): DG-N85 DG-N170, DG-N170+85, DG-N170+170; fertilisation with digestate together with Ni: DG-N85+Ni60, DG-N170+Ni60, DG-N170+Ni25. The results showed a soil organic amendment effect of the SF with a beneficial effect on SOM, soil pH and exchangeable bases. The SF was able to replace part of the mineral N fertilisation. The low mineralisation of the stable organic matter together with some immobilisation of mineral N from SF caused low N availability. The fertilisation planning should consider the SF ratio between the organic N (NO) and total N (TKN). Low NO:TKN ratios (≈0.65) needed lower Ni addition to maintaining the biomass production similar to the mineral fertilisation.

scholarly journals Dynamics of mineral nitrogen in topsoil, during regrowth of pasture in two contrasting grassland systems

1991 ◽  
pp. 203-208
Author(s):  
B.E. Ruz-Jerez ◽  
P.Roger Ball ◽  
R.E. White
Keyword(s):  
Mineral Nitrogen ◽  
Organic N ◽  
Soil Cores ◽  
Soil Mineral Nitrogen ◽  
Mineral N ◽  
Undisturbed Soil ◽  
Net Mineralisation ◽  
Organic Combination ◽  
Undisturbed Soil Cores ◽  
Intensive System

Changes in soil mineral nitrogen(N) were monitored during regrowth of pasture between consecutive grazings in two contrasting grassland systems; Grass-clover (the norm in NZ) and a more intensive system, Grass+N400 (pure grass + 400 kg fertiliser N/ha/year). The experiment was carried out during autumn at DSIR Grasslands.Palmerston North. Net mineralisation of N under field conditions was estimate_d- i~n- an ancillary experiment, using soil samples from undisturbed soil cores contained in PVC tubes. The dynamics of mineral N in soil were dominated by a 'pulse' of ammonium, observable soon after grazing. Nitrification proceeded rapidly thereafter. Mineral N in soil then progressively declined, much of it going into organic combination presumably through uptake by plants. Since nitrate formation in the soil is minimised by maximising the residence time of N in plant (organic) form, differentmanagementoptions(varyinginfrequency and intensity of defoliation) may have important influences, not only on pasture utilisation and production, but also on the management of mineral N in the soil-plant-animal complex. Tubes embedded in soil and incubated in the field have provided some additional, useful perspectives. There was only limitedevidence for significant net mineralisation of organic N throughout the period of regrowth. Analyses of individual soil cores demonstrated a sharp contrast between the pasture at large and the 10 - 15% of total area influenced by urine from the previous grazing, in terms of mineral N content. 'Averaging' these by bulking numerous cores into a composite sample can provide an accurate quantitative estimate of mineral N, which can be related to herbage uptake of N over the whole area. But if losses of N (by leaching or volatilisation) are disproportionate to the concentration of mineral N in affected and unaffected volumes of soil, then bulking samples and averaging will not be the most appropriate way to estimate these losses. The results of this study point to the importance of the urine of grazing ruminants as a N substrate for pasture regrowth in the absence of fertiliser N. At the same time, urine patches provide the main avenue for Nescape to the wider environment from developed pastures. Keywords mineral N, N in pastures, N cycling by animals

Plant uptake of nitrogen from the organic nitrogen fraction of animal manures: a laboratory experiment

2000 ◽  
Vol 134 (2) ◽  
pp. 159-168 ◽  
Author(s):  
D. R. CHADWICK ◽  
F. JOHN ◽  
B. F. PAIN ◽  
B. J. CHAMBERS ◽  
J. WILLIAMS
Keyword(s):  
N Mineralization ◽  
Organic N ◽  
Pig Slurry ◽  
Mineral N ◽  
N Content ◽  
Total N ◽  
N Supply ◽  
Ammonium N ◽  
Cow Slurry ◽  
N Fractions

Twenty slurries, 20 farmyard manures (FYM) and 10 poultry manures were chemically analysed to characterize their nitrogen (N) fractions and to assess their potential organic N supply. The organic N fraction varied between manure types and represented from 14% to 99% of the total N content. The readily mineralizable N fraction, measured by refluxing with KCl, was largest in the pig FYMs and broiler litters, but on average only represented 7–8% of the total N content. A pot experiment was undertaken to measure N mineralization from the organic N fraction of 17 of these manures. The ammonium-N content of the manures was removed and the remaining organic N mixed with a low mineral N status sandy soil, which was sown with perennial ryegrass (Lolium perenne L.). N offtake was used as a measure of mineralization throughout the 199 day experiment. The greatest N mineralization was measured from a layer manure and a pig slurry, where N offtake represented 56% and 37% of the organic N added, respectively. Lowest (%) N mineralization was measured from a dairy cow slurry (< 2%) and a beef FYM (6%). The mineralization rate was negatively related to the C[ratio ]organic N ratio of the ammonium-N stripped manures (P < 0·01, r = −0·63).

Estimating mineralisation of organic nitrogen from biosolids and other organic wastes applied to soils in subtropical Australia

Soil Research ◽  
2012 ◽  
Vol 50 (2) ◽  
pp. 91 ◽  
Author(s):  
Guixin Pu ◽  
Mike Bell ◽  
Glenn Barry ◽  
Peter Want
Keyword(s):  
Significant Proportion ◽  
Organic Amendments ◽  
Land Application ◽  
Organic N ◽  
N Addition ◽  
Ambient Conditions ◽  
Mineral N ◽  
Total N ◽  
Significant Difference ◽  
Biosolids Application

One major benefit of land application of biosolids is to supply nitrogen (N) for agricultural crops, and understanding mineralisation processes is the key for better N-management strategies. Field studies were conducted to investigate the process of mineralisation of three biosolids products (aerobic, anaerobic, and thermally dried biosolids) incorporated into four different soils at rates of 7–90 wet t/ha in subtropical Queensland. Two of these studies also examined mineralisation rates of commonly used organic amendments (composts, manures, and sugarcane mill muds). Organic N in all biosolids products mineralised very rapidly under ambient conditions in subtropical Queensland, with rates much faster than from other common amendments. Biosolids mineralisation rates ranged from 30 to 80% of applied N during periods ranging from 3.5 to 18 months after biosolids application; these rates were much higher than those suggested in the biosolids land application guidelines established by the NSW EPA (15% for anaerobic and 25% for aerobic biosolids). There was no consistently significant difference in mineralisation rate between aerobic and anaerobic biosolids in our studies. When applied at similar rates of N addition, other organic amendments supplied much less N to the soil mineral N and plant N pools during the crop season. A significant proportion of the applied biosolids total N (up to 60%) was unaccounted for at the end of the observation period. High rates of N addition in calculated Nitrogen Limited Biosolids Application Rates (850–1250 kg N/ha) resulted in excessive accumulation of mineral N in the soil profile, which increases the environmental risks due to leaching, runoff, or gaseous N losses. Moreover, the rapid mineralisation of the biosolids organic N in these subtropical environments suggests that biosolids should be applied at lower rates than in temperate areas, and that care must be taken with the timing to maximise plant uptake and minimise possible leaching, runoff, or denitrification losses of mineralised N.

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