Foreword
Tillage, mineralization and leaching
The nitrate problem has arguably been the most potent in¯uence on the direction of agro-ecological research in the developed world during the past 20 years. In UK, nitrate concentrations increased for two decades before levelling off in the 1980s, causing intense public concern about the effects on human health and the environment and stimulating a large programme of research. Similar trends stimulated research elsewhere in Europe and in North America. The pattern of increase in nitrate concentrations resembled that of the increase in consumption of nitrogen fertilizer, leading many to assume that there was a direct relation between fertilizer consumption and nitrate in water. This, however, was clearly an oversimpli®cation, because nitrate can be lost from the land even when no fertilizer is applied at all. Measure-ments made at Rothamsted showed that, even back in the 1870s, water draining from a soil that had received no nitrogen fertilizer for at least 10 years exceeded the present-day EC nitrate limit of 50 g mÿ3
. This nitrate was produced by the mineralization of nitrogen-con-taining organic matter in the soil.
The microbes that are active in mineralizing soil organic matter do so when the conditions suit them rather than when crops need nitrogen. They particu-larly enjoy the conditions which they ®nd when soil warm from the summer is rewetted during early autumn, but this is just when the nitrate they produce is most untimely. Arable soil is bare of vegetation at that time and rain will soon be percolating through it for 4±6 months. This naturally produced nitrate is usually responsible for a larger proportion of the nitrate leached from the soil than fertilizer nitrogen applied to growing crops in spring, so managing
mineralization is clearly one of the keys to managing the nitrate problem.
Roughly 70% of the nitrogen given to winter wheat crops, for example, is recovered in the harvested grain and straw. Stubble and roots remain in the soil together with any material exuded from the roots, and together they contain about 15% of the fertilizer in organic form. For winter wheat grown in UK, this 15% means that about 25±30 kg haÿ1
of nitrogen from fertilizer is added to the soil's organic nitrogen each season. This is hardly a matter for concern, given that nitrogen has also been mineralized from this source, but we need to remember that, although this fertilizer nitrogen has escaped leaching initially, it may become vulnerable when it is miner-alized in a subsequent season. We must therefore consider long-term trends.
Calculations made by ADAS in UK showed that up to 1976 farmers applied no more nitrogen to winter wheat than was removed by the crop. From 1977 onwards, however, more nitrogen was given to the crop than it removed, and by 1986 a total cumulative excess (of application over removal) of more than 300 kg haÿ1
had built up during the 10 years. Calcula-tions made by the present author suggested that about 45% of this excess, about 145 kg haÿ1
of nitrogen, remained in the soil as organic nitrogen in 1986. This excess has more than doubled since then, and it is likely that about 300 kg haÿ1
of fertilizer-derived nitrogen has now accumulated in soil organic matter. (This is not the same 300 kg haÿ1
as that identi®ed as the original excess in the ADAS calculations; the numerical coincidence is unfortunate.) The ®gure is for soils growing winter wheat in UK, but there have
Soil & Tillage Research 53 (2000) 163±165
probably been broadly similar accumulations in soils growing cereal crops at comparable levels of intensity elsewhere in the temperate zones.
An extra 300 kg haÿ1
of organic nitrogen in the soil is not necessarily a bad thing. The soil has become potentially more fertile, and roughly 3 Mg haÿ1
of carbon has been sequestrated with the nitrogen. The only problem is that some of the nitrogen will be remineralized. The calculations mentioned in the pre-vious paragraph suggest that this remineralization will be too slow to contribute signi®cantly to concentra-tions of nitrate in drainage from arable land, but we need to be aware of factors that could accelerate it. Among these is tillage.
Tillage is well known to enhance the mineralization of carbon and nitrogen in organic matter in the soil by exposing previously unexposed soil surfaces to oxy-gen and microbes and by providing the latter with new sources of energy. The literature on the topic goes back over 40 years. The purpose of tilling the soil depends on the agricultural system and the type of soil. The most basic is to achieve a seed-bed, but other important objectives include incorporating the resi-dues of the previous crop and burying weeds and their seeds. Tillage practices range from the self-explana-tory zero-tillage through minimum tillage, which breaks the soil surface but does little more, to more comprehensive systems that involve primary and sec-ondary tillage. Primary tillage usually inverts the soil, mainly to incorporate residues or bury weeds, while secondary tillage provides the seed-bed. These differ-ing systems vary considerably in the way they modify the soil structure and the extent to which they do so. They also differ in the amounts of energy they put into the soil. These differences have an impact on the microbes and soil animals which bring about miner-alization as they metabolize organic carbon and nitro-gen.
Tillage also alters the pathways for water in the soil. The aim may be to facilitate the passage of water from the soil or to encourage water storage, the former involving the creation of preferential pathways and the latter the establishment of a uniform tilth with good porosity. Untimely tillage can result in a compacted layer at the base of the plough that impedes water ¯ow. The effect of tillage on soil water also leads to a secondary effect on the soil microbes which are responsible for mineralization.
They are not happy in very dry or very wet soil, and tend to suspend their activities until conditions suit them better.
We can see overall that tillage in¯uences nitrate losses from the soil by encouraging mineralization and thence the production of nitrate in the soil and by altering the ¯ow pathways through which water car-ries nitrate from the soil. This is the rationale for the title and subject matter of this special issue, Tillage Mineralization and Leaching.
The ®rst two papers set the overall scene at the ®eld scale. Both describe experiments with tillage treat-ments in which water draining from the soil was sampled and analysed for nitrate and other chemicals of interest. The ®rst is an excellent account by Martin Shipitalo and his colleagues of research of this kind in USA, and the second describes research by John Catt and his co-workers at the Brimstone Farm site in UK. Both papers are central to the topic of this special issue and both include results from long-term experiments. There are, as you will see, some interesting differences in emphasis between them.
The next two papers are concerned with microbes and organic matter at a much smaller scale. Iain Young and Karl Ritz present a fascinating account of how tillage affects the working space of the soil microbes and other organisms that are involved in mineraliza-tion, nitri®cation and denitri®cation. Two issues that they discuss, spatial heterogeneity and the functional appraisal of soil structure seem fundamental to an understanding of what tillage actually does. JeÂroÃme Balesdent and his colleagues explain how the dynamics of organic matter are in¯uenced by physical protection and how tillage affects both the dynamics and the protection. They also suggest a mathematical model that describes how organic matter is incorpo-rated in aggregates, becomes protected from biode-gradation and contributes to the stability of the aggregates.
Tillage puts mechanical energy into the soil, and this stimulates the respiration of organic carbon that accompanies the mineralization of organic nitrogen. The paper by Chris Watts and his colleagues deals with measurements of respiration at both laboratory and ®eld scale. The experiments at both scales showed respiration to increase with the amount of energy put in, but only for a fairly short period in each case.
Modelling the effect of tillage on soil pathways for water is dif®cult. So is modelling nitrate leaching in cracked clay soils. The paper by Andy Matthews and colleagues takes on both problems simultaneously. The CRACK water-¯ow model has been adapted to include mineralization and was used to simulate nitrate leaching from the Brimstone Farm experiment mentioned above and investigate likely tillage effects on nitrate losses from the plots.
Nitrate is almost certainly not the threat to health it was once thought to be and is probably part of the body's defences againstSalmonellaand similar organ-isms. Also, it is phosphate, rather than nitrate, which limits the formation of algal blooms in fresh water, although nitrate remains the main culprit in seawater. There has therefore been a great upsurge in interest in phosphate losses from the soil, and the ®nal paper in the issue, by Tom Addiscott and Dawn Thomas, is a review of the possible effects of tillage on phosphate losses from the soil and of possible ways of preventing such losses.
This special issue was suggested by Chris Mullins. I am grateful to all the contributors for their efforts in producing the papers and for their patience with the delays in getting it out. These delays were mainly due to my organizational de®ciencies, but the ever-increasing bureaucratic load on researchers and the fact that I almost died half-way through the process did not help. Judith Taylor and her colleagues from Elsevier kindly took over the ®nal stages of the editorial work.
Accepted 6 December 1999 T.M. Addiscott*
Soil Science Department IACR, Rothamsted Harpenden Hertsfordshire AL5 2JQ UK
*
Tel.:44-1582-76-31-33; fax:44-1582-76-09-81.