Publisher Summary Soils form a major repository of nitrogen (N) within both natural and agricultural terrestrial ecosystems, containing, on a global basis, an estimated 2.4 x 10 11 tons of N. The soil receives N inputs through fertilizer additions and from the atmosphere in precipitation and dry deposition or via biological fixation; inputs are also made in plant and animal residues. N is removed in the harvested crop and is lost by leaching and surface run-off of soluble forms, by gaseous transfer as N gas and N oxides (during nitrification and denitrification processes), and by ammonia volatilization. In some circumstances, erosion may also be important. In addition to these interactions with the total ecosystem, internal cycles also operate within the soil, so that even if gains and losses are in balance, then N still continues to cycle in the soil. This chapter describes the current understanding of the conceptual basis of the processes involved in mineralization, relationships among the processes and other factors, and how their effects can be determined practically. The aim is to present this in a way that is relevant to current and future agricultural development and to environmental issues.

Nitrogen Mineralization in Temperate Agricultural Soils: Processes and Measurement

Publisher Summary This chapter discusses the effects of cultivation on soil-nitrogen mineralization The chapter provides extensive information on methods of mineralization measurement, cultivation effects on soil physical conditions and nitrogen mineralization, and management implications The chapter summarizes the available methods of measuring mineralization, assess the effects of cultivation on soil physical properties (which will influence biochemical processes), and review evidence for the effects of cultivation on nitrogen mineralization Within the review, one aims to quantify the size (or reported ranges) of cultivation effects on the mineral nitrogen pool, including the effects on the overall magnitude and temporal dynamics of nitrogen release; to examine the reasons for cultivation apparently increasing mineralization; and to provide information on areas where further research is required

The Effects of Cultivation on Soil Nitrogen Mineralization

In situ 15N techniques were used to quantify gross mineralisation, immobilisation and nitrification of N in bare soil nonamended and after incorporation of wheat straw (8 t ha−1). Experimental plots were confined within open-ended cylinders inserted into the tilled layer to 20 cm depth of a loamy soil (Typic Hapludalf). Measurements of gross N fluxes and C mineralisation were made during the year following incorporation of residues (September 1993–October 1994). At each date of measurement, inorganic N was added to soil by multiple injections of 15NH415NO3 (atom % excess=50.0) to a soil depth of 17.5 cm. The amount of NH4+-N, NO3−-N, organic N and their 15N isotopic composition were determined 30 min after 15N injections (t0), and 2–3 d after in situ incubation (t1). N transformation rates were estimated using a numerical model, Fluaz. In the nonamended soil, gross mineralisation in the 0–20 cm layer varied between 0.23 mg N kg−1 soil d−1 in December and 0.81 mg N kg−1 soil d−1 in July. Immobilisation rate varied similarly and was about one-third of gross mineralisation. Gross fluxes corrected for the effects of soil temperature and water content, and expressed as a function of normalised time (day at 10°C and optimal water potential) varied much less. The mean rate of gross mineralisation was 0.42 mg N kg−1 soil normalised d−1, and for immobilisation it was 0.13 mg N kg−1 soil normalised d−1, resulting in net mineralisation of 0.29 mg N kg−1 soil normalised d−1. Gross mineralisation and immobilisation were both increased by straw addition, with rates of 0.99 and 1.98 mg N kg−1 soil d−1 respectively, observed for the first measurement in September, 7 d after incorporation. Immobilisation and mineralisation decreased steadily with time. One year after straw addition, gross immobilisation and mineralisation rates were similar to those measured in nonamended soil. This was consistent with the changes in rate of straw-C mineralisation and with the depletion of the coarse fraction in soil, determined in parallel.

In situ changes in gross N transformations in bare soil after addition of straw

Summary

This paper shows how the wavelet transform can be used to analyse the complex spatial covariation of the rate of nitrous oxide (N2O) emissions from the soil with soil properties that are expected to control the evolution of N2O. We use data on N2O emission rates from soil cores collected at 4-m intervals on a 1024-m transect across arable land at Silsoe in England. Various soil properties, particularly those expected to influence N2O production in the soil, were also determined on these cores.



We used the adapted maximal overlap discrete wavelet transform (AMODWT) coefficients for the N2O emissions and soil variables to compute their wavelet covariances and correlations. These showed that, over the transect as a whole, some soil properties were significantly correlated with N2O emissions at fine spatial scales (soil carbon content), others at intermediate scales (soil water content) and others at coarse spatial scales (soil pH). Ammonium did not appear to be correlated with N2O emissions at any scale, suggesting that nitrification was not a significant source of N2O from these soils in the conditions that pertained at sampling.



We used a procedure to detect changes in the wavelet correlations at several spatial scales. This showed that certain soil properties were correlated with N2O emissions only under certain conditions of topography or parent material. This is not unexpected given that N2O is generated by biological processes in the soil, so the rate of emission may be subject to one limiting factor in one environment and a different factor elsewhere. Such changes in the relationship between variables from one part of the landscape to another is not consistent with the geostatistical assumption that our data are realizations of coregionalized random variables.

Scale- and location-dependent correlation of nitrous oxide emissions with soil properties: an analysis using wavelets

The Broadbalk Wheat Experiment at Rothamsted (UK) includes plots given the same annual applications of inorganic nitrogen (N) fertilizer each year since 1852 (48, 96 and 144 kg N/ha, termed N1 N2 and N3 respectively). These very long-term N treatments have increased total soil N content, relative to the plot never receiving fertilizer N (N0), due to the greater return of organic N to the soil in roots, root exudates, stubble, etc (the straw is not incorporated). The application of 144 kg N/ha for 135 years has increased total soil N content by 21%, or 570 kg/ha (0–23 cm). Other plots given smaller applications of N for the same time show smaller increases; these differences were established within 30 years. Increases in total soil N content have been detected after 20 years in the plot given 192 kg N/ha since 1968 (N4).There was a proportionally greater increase in N mineralization. Crop uptake of mineralized N was typically 12–30 kg N/ha greater from the N3 and N4 treatments than the uptake of c. 30 kg N/ha from the N0 treatment. Results from laboratory incubations show the importance of recently added residues (roots, stubble, etc) on N mineralization. In short-term (2–3 week) incubations, with soil sampled at harvest, N mineralization was up to 60% greater from the N3 treatment than from N0. In long-term incubations, or in soil without recently added residues, differences between long-term fertilizer treatments were much less marked. Inputs of organic N to the soil from weeds (principally Equisetum arvense L.) to the N0–N2 plots over the last few years may have partially obscured any underlying differences in mineralization.The long-term fertilizer treatments appeared to have had no effect on soil microbial biomass N or carbon (C) content, but have increased the specific mineralization rate of the biomass (defined as N mineralized per unit of biomass).Greater N mineralization will also increase losses of N from the system, via leaching and gaseous emissions. In December 1988 the N3 and N4 plots contained respectively 14 and 23 kg/ha more inorganic N in the profile (0–100 cm) than the N0 plot, due to greater N mineralization. These small differences are important as it only requires 23 kg N/ha to be leached from Broadbalk to increase the nitrate concentration of percolating water above the 1980 EC Drinking Water Quality Directive limit of 11·3mgN/l.The use of fertilizer N has increased N mineralization due to the build-up of soil organic N. In addition, much of the organic N in Broadbalk topsoil is now derived from fertilizer N. A computer model of N mineralization on Broadbalk estimated that after applying 144 kg N/ha for 140 years, up to half of the N mineralized each year was originally derived from fertilizer N.In the short-term, the amount of fertilizer N applied usually has little direct effect on losses of N over winter. In most years little fertilizer-derived N remains in Broadbalk soil in inorganic form at harvest from applications of up to 192 kg N/ha. However, in two very dry years (1989 and 1990) large inorganic N residues remained at harvest where 144 and 192 kg N/ha had been applied, even though the crop continued to respond to fertilizer N, up to at least 240 kg N/ha.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0021859600078527

The effects of long-term applications of inorganic nitrogen fertilizer on soil nitrogen in the Broadbalk Wheat Experiment.

A technique for measuring net rates of mineralization under field conditions is described. Soil cores were incubated in the field in sealed containers with acetylene to inhibit nitrification and thereby minimize losses of N through denitrification. Mineralization was estimated as the difference between the mineral N content after a 14-d incubation and that determined from soil samples taken at the start of incubation. Mineralization in the spring and summer in unfertilized plots in the field amounted to 90 and 70 kg N ha−1 in S.E. England under grass and grass/clover swards, respectively, and 40 kg N ha−1 under a grass sward in S.W. England. Daily rates of mineralization ranged from 0.02 to 1.90 kg N ha−1, with peak values related to re-wetting of the soil after dry weather. Laboratory incubation of soil showed that neither the low concentration of acetylene (2% v/v) adopted for field incubation, nor the accumulation of mineral N during incubation was likely to affect the total measurement, but that frequent and regular soil sampling was necessary to minimize the effects of changes in soil water content. Estimates for mineralization over the whole growing season (180 d) were obtained for two years from extrapolation of the early season field measurements and were, on average, 50% higher than predictions based on a chemical extraction index of potentially mineralizable N.

Lois Philipps

Papers

Field measurement of nitrogen mineralization using soil core incubation and acetylene inhibition of nitrification