K. W. T. Goulding

Gross nitrogen fluxes in soil : theory, measurement and application of 15N pool dilution techniques

Abstract Isotopic pool dilution using 15 N is proving to be a valuable tool for increasing our understanding of gross N cycling processes and our ability to both model these processes and link them to microbial function. However, not all applications are appropriate. Many of the questions asked by agronomists and soil scientists can often be addressed by simpler experiments in which measurements of the main parameters of inorganic and total N content of soil and plant components would suffice. In addition, the theory, assumptions and techniques associated with the calculation of gross N fluxes can lead to large errors if not applied correctly. Some preliminary assessment of the principle N transformation processes to be studied, followed by an optimisation of the experimental conditions are needed for the effective application of 15 N pool dilution. When applied correctly under carefully controlled laboratory incubations, the technique has been used successfully to quantify gross N fluxes and to understand the fundamental processes that regulate individual microbial N pathways. This has improved our understanding of how C and N cycles are linked, and thus has led us to question the most appropriate structure of C and N cycling models. Field based 15 N pool dilution studies have been used successfully to study the climatic influence on the soil N cycle and also to quantify the impact of external inputs. Further field-based studies are required to aid model development and evaluation. Linking soil microbial/molecular ecology with process-based studies of microbial nutrient cycling presents a new and exciting field of research that will benefit from the further application of isotopic pool dilution techniques for N and other nutrients.

Effects of atmospheric deposition, soil pH and acidification on heavy metal contents in soils and vegetation of semi-natural ecosystems at Rothamsted Experimental Station, UK

The effects of acidification on the soil chemistry and plant availability of the metals Pb, Cd, Zn, Cu, Mn and Ni in new and archived soil and plant samples taken from the >100-year-old experiments on natural woodland regeneration (Geescroft and Broadbalk Wildernesses) and a hay meadow (Park Grass) at Rothamsted Experimental Station are examined. We measured a significant input of metals from atmospheric deposition, enhanced under woodland by 33% (Ni) to 259% (Zn); Pb deposition was greatly influenced by vehicle emissions and the introduction of Pb in petrol. The build up of metals by long-term deposition was influenced by acidification, mobilization and leaching, but leaching, generally, only occurred in soils at pH<4. Mn and Cd were most sensitive to soil acidity with effective mobilization occurring at pH 6.0–5.5 (0.01 M CaCl2), followed by Zn, Ni and Cu at pH 5.5–5.0. Pb was not mobilized until pH<4.5. Acidification to pH 4 mobilized 60–90% of total soil Cd but this was adsorbed onto ion exchange surfaces and/or complexed with soil organic matter. This buffering effect of ion exchange surfaces and organic matter in soils down to pH 4 was generally reflected by all the metals investigated. For grassland the maximum accumulation of metals in herbage generally corresponded to a soil pH of 4.0. For woodland the concentration of Pb, Mn and Cd in oak saplings (Quercus robur) was 3-, 4- and 6-fold larger at pH 4 than at pH 7. Mature Oak trees contained 10 times more Mn, 4 times more Ni and 3 times more Cd in their leaves at pH 4 than at pH 7. At pH values <4.0 on grassland the metal content in herbage declined. Only for Mn and Zn did this reflect a decline in the plant available soil content attributed to long-term acid weathering and leaching. The chief cause was a long-term decline in plant species richness and the increased dominance of two acid-tolerant, metal-excluder species

Changes in soil phosphorus fractions following positive and negative phosphorus balances for long periods

The effect of phosphorus (P) balance (addition, in both fertilizers and farmyard manure (FYM), minus removal in crops) on eight soil P fractions determined by sequential extraction, was measured on archived soils from various long-term experiments run by Rothamsted Experimental Station in the United Kingdom. It has been established unequivocally that, for all the soils investigated, no one of the eight P fractions was increased or decreased during long periods of P addition or depletion, respectively. However, changes were mainly in the resin (24–30%) and the inorganic (Pi) component of the four fractions extracted sequentially by 0.5 M NaHCO3, 0.1 M NaOH, 1.0 M NaOH, 0.5 M H2SO4 (41–60%). For the sandy loam there were also consistent changes in the organic (Po) fraction (25%), especially that extracted by bicarbonate, presumably because the soil contained only a little clay and presumably had low sorption capacity. When the soils were cropped without P addition the largest proportional change was in the P extracted by resin, 0.5 M NaHCO3 and 0.1 M NaOH, suggesting that the P in these fractions is readily available, or has the potential to become available, for crop growth. This was supported by changes in the overall P balance. On the heavier textured soils, 50–80% of the change in total soil P (PT) was in these fractions; on the sandy soil this increased to more than 90%. The change in the sum of the first five fractions accounted, on average, for 90% of the P balance. However these changes in the P in the plough layer frequently left large amounts of P unaccounted for in some of the excessively P enriched soils. The amount of Pi extracted by resin and bicarbonate (Pi(r+b)) ranged between 14 and 50% of the sum of the Pi fractions. Soils with the lower percentages were those known to be most responsive to P fertilizers. Pi(r+b) accounted for an average of 70% of the P balance (negative) in P depleting soils where crop offtake was not offset or exceeded by annual P additions (positive balance). The ratio between Pi(r+b) and Pi(sum) could be a guide in defining soils deficient in P and those which are excessively enriched.

The use of cover crops in cereal-based cropping systems to control nitrate leaching in SE England

Field experiments were done to evaluate the extent to which cover crops can be used to help farmers comply with current legislation on nitrate leaching from arable land in nitrate vulnerable zones. Nitrate leaching was measured in sandy loam and chalky loam soils under a range of early sown (mid-August) cover crops at two sites in SE England, and in the subsequent winter following their incorporation. Cover crop species tested were forage rape, rye, white mustard, a rye/white mustard mixture, Phacelia and ryegrass. Additional treatments were weeds plus cereal volunteers, a bare fallow and a conventional winter barley crop sown one month later than the cover crops and grown to maturity. Cover crop and bare fallow treatments were followed by spring barley. This was followed by winter barley, as was the conventional winter barley crop. In the winter immediately after establishment, early sown cover crops decreased nitrate leaching by 29–91% compared to bare fallow. They were most effective in a wet winter on the sandy loam where nitrate leaching under bare fallow was greatest. There was little difference between cover crop species with respect to their capacity to decrease nitrate leaching, but losses were consistently smaller under forage rape. The growth of weeds plus cereal volunteers significantly decreased nitrate leaching on the sandy loam compared with a bare fallow, but was less effective on the chalky loam. Nitrate leaching under the later sown winter barley was often greater than under cover crops, but under dry conditions leaching losses were similar. In the longer-term, in most cases, the inclusion of cover crops in predominantly cereal-based cropping systems did not significantly decrease cumulative nitrate leaching compared with two successive winter cereals. In summary, early sown cover crops are most likely to be effective when grown on freely drained sandy soils where the risk of nitrate leaching is greatest. They are less likely to be effective on poorer drained, medium-heavy textured soils in the driest parts of SE England. In these areas the regeneration of weeds and cereal volunteers together with some additional broadcast seed may be sufficient to avoid excessive nitrate losses. In the short-term, mineralization of N derived from the relatively small cover crops grown once every 3–4u2009years in cereal-based cropping systems is unlikely to contribute greatly to nitrate leaching in later years and adjustments to fertilizer N recommendations will not usually be necessary.

Nitrous Oxide Emissions from Two Riparian Ecosystems: Key Controlling Variables

Nitrous oxide (N2O) emissions were measured weekly to fortnightly between April 2001 and March 2002 from two riparian ecosystems draining different agricultural fields. The fields differed in the nature of the crop grown and the amount of fertiliser applied. Soil water content and soil temperature were very important controls of N2O emission rates, with a ‘threshold’ response at 24% moisture content (by volume) and 8 °C., below which N2O emission was very low. N2O fluxes were higher at the site that had received the most fertiliser N, but NO 3 − was not a limiting factor at either site. There was also a ‘threshold’ effect of rainfall, in which major rainfall events (≥10 mm) triggered a pulse of high N2O emission if none of the other environmental factors were limiting. These results suggest the existence of multiple controls on N2O emissions operating at a range of spatial and temporal scales and that non-linear relationships, perhaps with a hierarchical structure, are needed to model these emissions from riparian ecosystems.

Nitrogen Management for Profitable Farming with Minimal Environmental Impact: The Challenge for Mixed Farms in the Cotswold Hills, England

A detailed nitrogen (N) budget was constructed for a mixed farm in the Cotswold Hills, England, situated on thin, well drained soils prone to leaching. The study covered all stages of the farms seven-year rotation and included the removal of the dairy herd. All inputs and outputs of N were measured or estimated and a balanced budget achieved, but only by including relatively expensive measurements of soluble organic nitrogen (SON) leached. Leaching was the main loss process. Given the nature of the soil and the influence of the weather, it would be difficult to reduce losses without drastic reductions in fertiliser inputs or stocking rates. Nitrogen use efficiency averaged 46%. The mean N surplus declined from 141 kg N ha−1 to 117 kg N ha−1 with the removal of the dairy herd. However, the farm to which the herd moved had an N surplus of 392 kg N ha−1. Simple farm gate N budgets were constructed for neighbouring Cotswold farms to encourage farmers to consider ways to improve N use. Implications for policy to reduce losses of N while maintaining farm profitability are discussed.

Perspectives and Challenges in the Future Use of Plant Nutrients in Tilled and Mixed Agricultural Systems

Abstract Producing an adequate quantity of healthy food without polluting the environment is a serious challenge for future agriculture around the world. The Food 21 research program in Sweden has researched all aspects—economic, environmental, and social—of sustainable farming systems. This paper presents some of the research from that and other relevant international research programs that have focused on better nutrient-use efficiency, especially for nitrogen and phosphorus. It shows that a range of sustainable solutions to nutrient-use efficiency exists, some of which are complex but some very simple. Government policies, including subsidies; research and technology; and public acceptance of farming practices all combine to create these solutions. Participatory approaches to knowledge transfer are needed, in which scientists, policy makers, farmers, advisers, and consumers exchange information and together build sustainable farming systems.