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Abstract Phosphorus (P) is central to the productivity of grass-clover pastoral farming systems. Fertiliser P promotes the growth of clover, which provides an input of fixed N to complement that supplied from the soil (primarily mineralised N). If pasture productivity is limited by P availability, organic matter returns to the soil in excreta and plant residues will decline, which in turn may reduce the supply of N by mineralisation. We examined the effect of long-term P application to grass-clover pasture on mineralisation of N (and C). Net N mineralisation was measured in a 14-week aerobic incubation (25 °C; soil maintained at field capacity) using soils (0–15 cm depth) from a long-term (1952–2016) trial set up to quantify effects of single superphosphate (0, 188, or 376 kg−1 year−1) on productivity of irrigated, sheep-grazed pasture (no fertiliser N was applied during the trial). Although P fertilisation had only a small effect (∼10% increase) on total soil N, net N mineralisation was substantially increased (N mineralised from fertilised soil in 14 weeks was ∼1.6 times that from the unfertilised Control). In contrast, mineralisation of C was slightly greater in the Control than in fertilised soil. Nitrogen mineralisation exhibited a Mitscherlich-type relationship with available soil P, measured using the Olsen test; near-maximum mineralisation was observed at an Olsen test value of ∼10 mg P kg−1 soil. Annual in-field N mineralisation was estimated by modifying the laboratory-measured "basal" mineralisation values using temperature and moisture adjustment factors (soil temperature and moisture data were acquired from an adjacent, irrigated trial). The results confirmed that N mineralisation was the predominant source of available N in the unfertilised Control (∼160 kg ha−1 yr−1 vs ∼30 kg ha−1 yr−1 of fixed N) and that it was an important source of N for the additional dry matter grown in response to P application (N uptake increased by ∼200 kg ha−1 yr−1 in response to P fertilisation vs an increase in mineralisation of ∼100 kg ha−1 yr−1). The increase in net N mineralisation in fertilised soil was partly because immobilisation of N was less than in the unfertilised Control. Highlights • Effect of P fertilisation of ryegrass-clover pasture on soil N supply was assessed. • After 64 years, net N mineralisation was increased by up to 66% by P application. • The response of ryegrass to P was largely due to increased N availability. • P-induced increase in N mineralisation was partly due to increase in soil organic N. • However, the effect of P on N supply was mainly due to decreased immobilisation of N. [ABSTRACT FROM AUTHOR]

Nitrous oxide emitted from urine patches is a key source of agricultural greenhouse gas emissions. A better understanding of the complex soil environmental and biochemical regulation of urine-N transformations in wet soils is needed to predict N2O emissions from grazing and also to develop targeted mitigation technologies. Soil aeration, gas diffusion and drainage are key factors regulating N transformations and are affected by compaction during grazing. To understand how soil compaction from animal treading influences N transformations of urine in wet soils, we applied pressures of 0, 220 and 400 kPa to repacked soil cores, followed by 15N-labeled synthetic urine, and then subjected the cores to three successive saturation-drainage cycles on tension tables from 0 to 10 kPa. Compaction had a relatively small effect on soil bulk density (increasing from 0.81 to 0.88 Mg m−3), but strongly affected the pore size distribution. Compaction reduced both total soil porosity and macroporosity. It also affected the pore size distribution, principally by decreasing the proportion of 30-60 μm and 60-100 μm pores and increasing the proportion of micropores (<30 μm). Rates of urine-N transformations, emissions of N2 and N2O, and the N2O to N2 ratio were affected by the saturation/drainage cycles and degree of compaction. During the first saturation-drainage cycle, production of both N2O and N2 was low (<0.4 mg N m−2 h−1), probably because of anaerobic conditions inhibiting nitrification. In the second saturation/drainage cycle, the predominant product was N2 at all compaction rates. By the third cycle, with increasing availability of mineral-N substrates, N2O was the dominant product in the uncompacted (max = 4.70 mg N m−2 h−1) and 220 kPa compacted soils (max = 7.65 mg N m−2 h−1) with lower amounts of N2 produced, while N2 was produced in similar quantities to N2O (max = 3.11 mg N m−2 h−1) in the 400 kPa compacted soil. Reduced macroporosity in the most compacted soil contributed to more sustained N2 and N2O production as the soils drained. In addition, compaction affected the rate of change of soil pH and DOC, both of which affected the N2O to N2 ratio. Denitrification during drainage and re-saturation may make a large contribution to soil N2O emissions. Improving soil drainage and adopting grazing management practices that avoid soil compaction while increasing macroporosity will reduce total N2O and N2 emissions. [ABSTRACT FROM AUTHOR]

Water-extractable organic matter is regarded as readily available substrate for soil microbes. However, little is known about the influence of extraction temperature and duration on its biochemical composition. The effect of temperature (20 vs. 80 °C) and extraction duration (1-24 h) on the amounts of water-extractable organic C (WEOC), water-extractable organic N (WEON), carbohydrates, phenolics, ninhydrin-reactive organic N (NRON), and mineral N were evaluated using four agricultural soils from New Zealand and eastern Canada. More WEOC and WEON were extracted in hot than in cold water, and the same was found for all biochemical compounds tested. Biochemical components increased rapidly during the first 4 h of extraction at 80 °C, and at a slower rate thereafter. In contrast, at 20 °C, concentrations of all measured components were at or close to their maximum after 1 h and showed little change thereafter. Glucose and nitrate in the 20 °C extracts both declined substantially between 1 and 24 h, likely due to microbial activity. Moreover, the disappearance of glucose induced a decline in the ratio of carbohydrate-C to phenolic-C. The carbohydrates measured in water extracts at both temperatures were predominantly of microbial origin after 1 h. However, the proportion of microbial carbohydrates gradually declined and the proportion of plant carbohydrates increased from 1 to 24 h at 80 °C. Based on these findings, we recommend limiting extractions to 1 h or less at 20 °C and to 4 h or less at 80 °C to minimize compositional changes that may occur during longer extraction periods. [ABSTRACT FROM AUTHOR]

Abstract: Transformation of ruminant urine-nitrogen (N) can contribute to negative environmental outcomes such as nitrate leaching which leads to eutrophication of waterways and production of nitrous oxide (N2O), a greenhouse gas. Although abiotic factors influencing urine-N processing have been well studied, detailed studies of the soil microbial community dynamics following urine application are required to improve mitigation strategies for reducing harmful N fluxes from urine deposition. A factorial laboratory experiment using packed silt-loam soil cores with two levels each of urea (±), soil matric potential (ψ −6.0 or −0.2 kPa) and soil bulk density (ρ b 1.1 or 1.5 g cm−3) was performed to study the interaction of urea and soil physical conditions on both soil inorganic N transformations and the abundance of ammonia-oxidizers and denitrifiers. Soil ψ and ρ b treatments had an immediate impact on soil pH, dissolved organic carbon, inorganic N pools and emissions of N2O and N2 following urea deposition, and microorganisms carrying the nosZ gene were present in lower numbers in the most aerobic soil (−6.0 kPa and 1.1 g cm−3) from day 7. In all treatments, both bacterial amoA and denitrifier nirS, nirK and nosZ gene copy numbers increased within 1 day following urea application. Dynamics in the concentrations were significantly correlated with cumulative changes in the abundance of the ammonia-oxidizers, but no relation was found between cumulative changes in the denitrifier nirS, nirK and nosZ gene copy numbers and the dynamics in soil inorganic N, N2O or N2 emissions. Throughout most of the study period the specific soil conditions, induced by the ψ and ρ b treatments, determined nitrifier and denitrifier activity rather than the size of the microbial communities involved. However, by day 35 soil ψ and ρ b treatments exerted large treatment effects on bacterial amoA, nirS and nirK gene copy numbers. Thus, although nitrate concentrations and N2O emissions following urea deposition were determined by the soil ψ and ρ b conditions in the short-term, our results indicate that changes in the population sizes of denitrifiers and AOB in ruminant urine patches may influence environmental N fluxes in the long-term. [Copyright &y& Elsevier]