l-Methionine-sulphoximine affects N mineralisation-immobilisation in soil

Productivity of 3 different 2-year crop rotations, namely continuous wheat, wheat-chickpea, and wheat-fallow, was measured over 4 consecutive seasons beginning in 1991-92 at the ICARDA station, Tel Hadya, Syria. Nitrogen (N) fertiliser (30 kg N/ha at sowing) was broadcast every other year in the continuous wheat only. 15N-labelled fertiliser was used to quantify the amount of nitrogen supplied to the crops through current and past applications of fertiliser and by N2 fixation. The remaining N in the crop was assumed to come from the soil. In any single season, wheat yields were unaffected by rotation or N level. However, 2-year biomass production was significantly greater (32%, on average) in the continuously cropped plots than in the wheat-fallow rotation. On average, <10% of the N in the wheat crop came from fertiliser in the season of application, and <1·2 kg N/ha of the residual fertiliser was recovered by a subsequent wheat crop. Chickpea fixed 16-48 kg N/ha, depending on the season, but a negative soil N budget was still likely because the amount of N removed in the grain was usually greater than the amount of atmospheric N2 fixed. Uptake of soil N was similar in the cereal phase of all 3 rotations (38 kg N/ha, on average), but over the whole rotation at least 33% more soil N was removed from continuously cropped plots than from the wheat-fallow rotation, suggesting that the latter is a more sustainable system. A laboratory study showed that although wheat and chickpea residues enhanced the gross rate of N mineralisation by c. 50%, net rates of N mineralisation were usually negative. Given the high C/N ratio of the residue, immobilisation, rather than loss processes, is the likely cause of the decline in the mineral N content of the soil. Consequently, decomposition of crop residues in the field may in the short term reduce rather than increase the availability of N for crop growth.

Download Full-text

The fate of residual nitrogen fertiliser applied to a ryegrass (Lolium perenne L.) seed crop

Australian Journal of Agricultural Research ◽

10.1071/ar99085 ◽

2000 ◽

Vol 51 (2) ◽

pp. 287 ◽

Cited By ~ 5

Author(s):

W. R. Cookson ◽

J. S. Rowarth ◽

K. C. Cameron

Keyword(s):

Microbial Biomass ◽

Microbial Biomass N ◽

Root Mass ◽

N Mineralisation ◽

Pasture Production ◽

Total N ◽

Leaching Losses ◽

Seed Harvest ◽

Fertiliser Application ◽

Seed Crops

Large amounts of the nitrogen (N) fertiliser applied to ryegrass seed crops remain within the soil at seed harvest and can potentially affect subsequent pasture production and environmental contamination. The fate of residual urea-15N-labelled fertiliser and the effect of previous fertiliser application on subsequent leaching losses and pasture production was assessed during a 9-month period after seed harvest using monolith lysimeters (diameter, 180 mm; length, 300 mm) in Canterbury, New Zealand. Results indicated that leaching losses and pasture uptake of residual 15N-labelled fertiliser were largely restricted by the immobilisation of 15N-labelled fertiliser into soil organic pools and the expanding root mass. Most of the 15N-labelled fertiliser remaining in the soil 9 months after the seed harvest was present within the humified organic matter (50%) and microbial biomass (40%) pools; the majority (62%) was anaerobically mineralisable. The 15N-labelled fertiliser that became available was largely recovered in rapidly expanding ryegrass roots, which increased 3–4-fold between seed harvest (December 1997) and pasture harvest (September 1998). Root mass, soil mineral N, and soil microbial biomass N were significantly (P < 0.05) greater in fertilised treatments than in controls at pasture harvest; clay-fixed N, anaerobically mineralisable N, and total N were not affected. The results indicated that, in the short term, N mineralisation rates were increased by previous fertiliser application but there was little evidence of a longer term effect on N mineralisation rates.

Download Full-text

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

Soil Research ◽

10.1071/s97102 ◽

1998 ◽

Vol 36 (3) ◽

pp. 429 ◽

Cited By ~ 10

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.

Download Full-text