Nitrate present in arable soils in autumn is at risk to leaching during the following winter. To see whether unused nitrogen fertiliser was a major source of this nitrate, 15N-labelled fertiliser was applied to 11 winter wheat crops at rates of between 47 and 234 kg N ha−1in spring. The experiments were on three contrasting soil types in south-east England.



On average, 17′% of the N from spring-applied labelled fertiliser remained in the 0–23 cm soil layer at harvest (range, 7–36%) but only a small proportion was in inorganic forms (ammonium + nitrate). This was never more than 5 kg N ha−1and averaged only 1·3% of the fertiliser N applied (range, 0·4–3·6 %). Between 79 and 98% of the inorganic N in soils at harvest was unlabelled, being derived from the mineralisation of organic N rather than from unused fertiliser. The amount of unlabelled N was much greater where wheat was grown after ploughing up grass or grass/clover leys than where it was grown in all-arable rotations. When wheat was grown without N fertiliser, soil inorganic N content at harvest was no lower than in plots given fertiliser at rates up to 234 kg N ha−1.



This work indicates that, for soil growing winter wheat, almost all of the nitrate at risk to leaching over the winter period comes from mineralisation of organic N, not from unused fertiliser applied in spring. Consequently, even a drastic reduction in N fertiliser use would have little effect on nitrate leaching.

Unused fertiliser nitrogen in arable soils: its contribution to nitrate leaching

In this review, which consists of two parts, major interactions between nitrous oxide (N2>O) and soil solution are described. In the first part, as an introduction, concentrations of dissolved N2>O in different aqueous systems are summarized. An inventory of data on maximal N2>O concentrations in soil solution (up to 9984 μg N2>O-N l−1>) and in soil air (up to 8300 ppm) from literature is presented. The peak N2>O concentrations represent a N2>O supersaturation in the soil solution up to 30000 times with respect to ambient air and a soil air N2>O concentration about 25000 times higher than in the atmosphere. The main physico–chemical parameters (solubility, diffusion) controlling N2>O distribution between soil solution and soil air are outlined. The influences of cultivation practice, nitrogen turnover, water content and temperature on N2>O a ccumulation in soil solution and soil air are reviewed. In the second part some models of N2>O dynamics in soils are discussed with emphasis on N2>O transport processes. A simple qualitative scheme is developed to categorize the effects of the soil solution on N2>O dynamics in soils. In this scheme the temporary, intensive N2>O oversaturation of the soil solution is interpreted as a result of gas diffusion inhibition by water (barrier function of soil solution) resulting in an accumulation of N2>O. In addition, N2>O supersaturation is an indication that transitory much N2>O can be stored in the soil solution (storage function of soil solution). Where the soil solution flows up-, down- or sidewards it can act as a relevant transport medium for dissolved N2>O (transport function of soil solution). This scheme is applied to examples from the literature.

Effects of soil solution on the dynamics of N2O emissions: a review

Influence of improved subsurface drainage on phosphorus losses and nitrogen leaching from a heavy clay soil

Publisher Summary This chapter focuses on the linkage between subsurface tile drainage of agricultural lands and nitrate in surface waters, and the effect of uncontrollable factors and controllable factors on nitrate losses to subsurface drainage. Subsurface drainage is a common water management practice in highly productive agricultural areas with poorly drained soils that have seasonally perched water tables or shallow groundwater. This management practice increases crop productivity, reduces risk, and improves economic returns to crop producers. Agricultural drainage water has been identified as a major contributor to the nitrate-N loading of receiving waters. Research conducted at widely different scales of watershed basins point to the fact that agricultural systems do affect nitrate levels in river waters. Long-term, subsurface drainage research, which integrates the effects of climatic variability, soil properties, and various cropping systems, is vital to understanding of nitrate losses to subsurface drainage.

Nitrate Losses to Surface Water Through Subsurface, Tile Drainage

The main features of the climate and soils of the main crop production areas of different countries in north-west Europe are outlined, and the main crops grown are summarised. The need for tillage is considered; provided that suitable conditions exist for germination, root growth is most likely to be influenced by mechanical impedance and aeration. Traditional tillage may not always be necessary and may hinder timely sowing of crops. Shallow tillage can facilitate planting a larger area of crops closer to the optimum dates in autumn and in spring, which also saves energy and labour. Research experience with reduced tillage over the last 20–25 years is reviewed. Shallow tillage has been as good as mouldboard ploughing at the traditional depth in most countries on a wide range of soil types, if weed control was satisfactory. Direct drilling has often proved satisfactory in the absence of crop residues for autumn sown crops (wheat, barley, oil-seed rape and beans) where drainage (and therefore soil aeration) was not limiting. Direct drilling has not generally been successful for spring sown crops. Direct drilling is inappropriate for root crops, but strip tillage for sugar beet may become practicable. Some research on soil conditions associated with these experiments is considered. Crop residues are a major constraint to shallow tillage or direct drilling of autumn sown crops. The limitations of reduced tillage due to inadequate weed control, wheel-traffic damage and unsatisfactory drainage are reviewed. The need for occasional deep loosening is also discussed. Where the amount of residues is small, much tillage may be unnecessary, except to relieve compaction by wheels; bed systems or wide-span gantries to avoid wheel damage could become practical developments for some crops.

Reduced tillage in north-west Europe—A review

The soil water regimes, flow paths of water and concentrations of nutrients in this water were measured for a clay soil growing winter wheat in 1978–9 and 1979–80. The soil was either drained with mole drains at 2 m spacing connected to plot drains 46 m apart or undrained. In the 1st year a compacted layer at about 20 cm depth caused a perched water table in the Ap horizon in both drainage treatments, and prevented the mole drains at 60 cm from affecting the water table. In 1979–80 after cultivation to disrupt the compacted layer, midway between the mole drains the depth to the winter water table was 20 cm greater than in undrained soil.Surface flow, interflow at the depth of the plough layer and deep drainflow from mole and pipe drains responded rapidly to winter rainfall events. During both winters the mole and tile system removed most of the rainfall on the drained plots and the peaky hydrographs were typical of a mole system in a clay soil. In the undrained plots only a small proportion of the winter rainfall was accounted for in flow from the top 30 cm, and up to 75% of the water was able to percolate downwards possibly to below the barriers that separated the plots. Long-term water-balance studies indicated that a proportion of the water moving to depth in the undrained plots was probably entering the deep drainage system of the drained plots. As a result, the mole and pipe drainage system often removed more water than the rainfall input less evapotranspiration. This problem did not affect the depth to the water tables.For each flow component concentrations of nitrate, ammonium, nitrous oxide, phosphorus, potassium and calcium were measured in the drainage water. Concentrations of nitrate-N from all drained plots were largest in autumn, being in the range 50–95 mg N/1, but then decreased to 1–5 mg N/1 by the end of March. Losses of nitrate-N were mainly through the mole drains and amounted to 43·6 and 59·7 kg N/ha in the 2 years. The quantities of nitrate-N lost in surface runoff or in flow in the cultivated layer were small on both treatments. Gaseous nitrous oxide, ammonium and phosphorus contents were very small. Potassium concentrations were somewhat larger, but not exceeding 3·5 mg/1. The calcium concentrations were in the range 40–210 mg/1. Concentrations of herbicides measured in November 1980 were negligible.In the 2nd year water was taken up from a greater depth in the drained than in the undrained plots from April onwards. These results are discussed in relation to water supply to the crops at this site.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0021859600042118

A study of mole drainage with simplified cultivation for autumn-sown crops on a clay soil: 2. Soil water regimes, water balances and nutrient loss in drain water, 1978–80

P. Morgan

Papers

A study of mole drainage with simplified cultivation for autumn-sown crops on a clay soil: 2. Soil water regimes, water balances and nutrient loss in drain water, 1978–80