Soluble organic nitrogen in agricultural soils

 The existence of soluble organic forms of N in rain and drainage waters has been known for many years, but these have not been generally regarded as significant pools of N in agricultural soils. We review the size and function of both soluble organic N extracted from soils (SON) and dissolved organic N present in soil solution and drainage waters (DON) in arable agricultural soils. SON is of the same order of magnitude as mineral N and of equal size in many cases; 20–30 kg SON-N ha^–1 is present in a wide range of arable agricultural soils from England. Its dynamics are affected by mineralisation, immobilisation, leaching and plant uptake in the same way as those of mineral N, but its pool size is more constant than that of mineral N. DON can be sampled from soil solution using suction cups and collected in drainage waters. Significant amounts of DON are leached, but this comprises only about one-tenth of the SON extracted from the same soil. Leached DON may take with it nutrients, chelated or complexed metals and pesticides. SON/DON is clearly an important pool in N transformations and plant uptake, but there are still many gaps in our understanding. Received: 10 June 1999     

Short-term effects of nitrogen on methane oxidation in soils

 The short-term effects of N addition on CH₄ oxidation were studied in two soils. Both sites are unfertilized, one has been under long-term arable rotation, the other is a grassland that has been cut for hay for the past 125 years. The sites showed clear differences in their capacity to oxidise CH₄, the arable soil oxidised CH₄ at a rate of 0.013 μg CH₄ kg^–1 h^–1 and the grassland soil approximately an order of magnitude quicker. In both sites the addition of (NH₄)₂SO₄ caused an immediate reduction in the rate of atmospheric CH₄ oxidation approximately in inverse proportion to the amount of NH₄ ⁺ added. The addition of KNO₃ caused no change in the rate of CH₄ oxidation in the arable soil, but in the grassland soil after 9 days the rate of CH₄ oxidation had decreased from 0.22 μg CH₄ kg^–1 h^–1 to 0.13 μg CH₄ kg^–1 h^–1 in soil treated with the equivalent of 192 kg N ha^–1. A ¹⁵N isotopic dilution technique was used to investigate the role of nitrifiers in regulating CH₄ oxidation. The arable soil showed a low rate of gross N mineralisation (0.67 mg N kg^–1 day^–1), but a relatively high proportion of the mineralised N was nitrified. The grassland soil had a high rate of gross N mineralisation (18.28 mg N kg^–1 day^–1), but negligible nitrification activity. It is hypothesised that since there was virtually no nitrification in the grassland soil then CH₄ oxidation at this site must be methanotroph mediated. Received: 31 October 1997     

Soil acidification during more than 100 years under permanent grassland and woodland at Rothamsted

Abstract Soil samples have been taken periodically from unlimed plots of the 130-year-old Park Grass Experiment and from the 100-year-old Geescroft Wilderness at Rothamsted. Changes in the pH of the samples show how acidification has progressed. The soils are now at, or are approaching, equilibrium pH values which depend on the acidifying inputs and on the buffering capacities of the soils. We have calculated the contributions to soil acidification of natural sources of acidity in the soil, atmospheric deposition, crop growth and nutrient removal, and, where applicable, additions of fertilizers. The relative importance of each source of acidification has changed as the soils have become more acid. Acid rain (wet deposited acidity) is a negligible source, but total atmospheric deposition may comprise up to 30% of acidifying inputs at near neutral soil pH values and more as soil pH decreases. Excepting fertilizers, the greatest causes of soil acidification at or near neutral pH values are the natural inputs of H⁺ from the dissolution of CO₂ and subsequent dissociation of carbonic acid, and the mineralization of organic matter. Under grassland, single superphosphate and small amounts of sodium and magnesium sulphates have had no effect on soil pH, whilst potassium sulphate increased soil acidity slightly. All of these effects are greatly outweighed under grassland, however, by those of nitrogen fertilizers. Against a background of acidification from atmospheric, crop and natural inputs, nitrogen applied as ammonium sulphate decreased soil pH up to a maximum of 1.2 units at a rate in direct proportion to the amount added, and nitrogen applied as sodium nitrate increased soil pH by between 0.5 and 1 unit.     

Nitrate leaching losses and their control in a mixed farm system in the Cotswold Hills, England

Abstract. Nitrate leaching was measured for four years at the Royal Agricultural College 's Coates Farm in the Cotswolds, England. Coates is a typical Cotswold mixed farm with thin, well-drained calcareous soils especially prone to leaching. Over the duration of this study there were dairy, sheep and arable enterprises on the farm. A 'Farm Gate' nitrogen (N) budget was constructed. Small 120 m × 20 m 'farmlets' were sited in ten fields across the farm, covering all parts of the rotation, as the sites for detailed measurements. Each farmlet received the same management as the rest of the field in which they were situated. Using ceramic probes inserted to 60 cm, soil water was sampled every two weeks throughout the winter drainage season. The annual drainage varied from 135 mm under grassland in 1996/7 to 600 mm under cereals in 1998/9. Average N losses by leaching were determined mostly by rainfall and were 65 kg N ha^–1 yr^–1, accounting for 25% of the N inputs. Especially leaky parts of the rotation were the ploughing out of a lucerne ley and the grazing of stubble turnips with sheep, both typical Cotswold farm practices. The research highlights some of the difficulties in developing practicable, profitable management practices to decrease nitrate losses.     

Nitrogen deposition to land from the atmosphere

Abstract. One direct measurement and two indirect estimates suggest that 35–40 kg nitrogen per hectare are deposited on arable land from the atmosphere each year in the south and east of England. This could contribute markedly to nitrate leaching and soil acidification. It may also change the flora and fauna of 'natural' ecosystems, as such amounts are likely to exceed the critical load.     

Mobilization of aluminium in soil by acid deposition and its uptake by grass cut for hay – a Chemical Time Bomb

Abstract. Analyses of soil and hay samples collected from the Park Grass Experiment at Rothamsted during the last 137 years indicate slow but significant increases in KCl- and EDTA-extractable aluminium in soil and a sudden and very large recent increase in the concentration of aluminium in the herbage. The latter is associated with a sudden increase in the rate of acidification of the soil over the last 10–15 years and the mobilization of aluminium as the soil enters the aluminium buffer range -a Chemical Time Bomb. Such severe acidification from atmospheric inputs on a well-buffered soil illustrates how quickly an apparently stable situation can change as a result of acid deposition. It highlights the need to protect soils and plants from the effects of acidification by decreasing acid inputs or by liming.     

Ecosystem‐based management of Amazon fisheries and wetlands

Infrastructure development and overfishing in the Amazon make it imperative to define adequate scales for the ecosystem‐based management of commercial fisheries and the wetlands on which they depend. We mapped fisheries and fish ecology data from Brazil, Peru, Bolivia and Colombia to an explicit GIS framework of river basins and mainstems. Migratory species account for more than 80% of the known maximum catches of commercial fisheries across the Amazon. Of these migratory species, we nominated six long‐distance migratory fish taxa as flagship species to define the two main commercial fishery regions. The migrations of at least one goliath catfish species define a large‐scale longitudinal link joining the Andes, Amazon Lowlands and Amazon River estuary. Migratory Characiforms demonstrate interbasin wetland connectivity between nutrient‐rich and nutrient‐poor rivers over at least 2 million km², or about one‐third of the Amazon Basin. We show that flooded forest area is the most important wetland variable explaining regional variations in migratory characiforme biomass as indicated by maximum annual fishery catches. The sustainable management of Amazon fisheries will require transnational cooperation and a paradigm shift from local community management alone to a more integrated approach that considers both rural and urban consumers and challenges, and the realistic life histories of migratory species.     

Temporal changes in chemical properties of air‐dried stored soils and their interpretation for long‐term experiments

The usefulness of stored soils from long‐term experiments is often questioned because of changes that might occur during storage. We examined changes during long‐term storage (8–69 years) in the chemical properties of soils with a range of pH values (3.4–8.1 in water) from woodland and grassland experiments at Rothamsted Experimental Station in the UK. No significant changes during storage were measured for total C and N. Large but erratic changes in exchangeable Na⁺ content between 1959 and 1991 were probably caused by contamination of the 1959 samples by perspiration and from sodium‐based glassware. Exchangeable K⁺ increased during storage but only by a small amount. Small changes in exchangeable Ca²⁺ and Mg²⁺ were measured in some samples but not in others. Generally the amount of exchangeable cations increased slightly during storage. This is probably linked to the decreases of 0.4 units in the pH of acid soils, which we attribute to the hydrolysis of approximately 0.25% of the exchangeable Al³⁺. A doubling of the amount of exchangeable Mn²⁺ during storage for 32 years was probably caused by re‐equilibration of Mn species. The most practicable way to prepare soil samples for long‐term storage is to dry them in air. However, those who study changes in soil by re‐analysing samples of the soil stored for a long time must (i) use the same methods of analysis, or (ii) demonstrate that different methods lead to the same results, and (iii) know what changes can arise during storage.     

Dual isotope and isotopomer measurements for the understanding of N2O production and consumption during denitrification in an arable soil

The aim of our research was to obtain information on the isotopic fingerprint of nitrous oxide (N₂O) associated with its production and consumption during denitrification. An arable soil was preincubated at high moisture content and subsequently amended with glucose (400 kg C ha^?1) and KNO₃ (80 kg N ha^?1) and kept at 85% water‐filled pore space. Twelve replicate samples of the soil were incubated for 13 days under a helium‐oxygen atmosphere, simultaneously measuring gas fluxes (N₂O, N₂ and CO₂) and isotope signatures (δ¹⁸O‐N₂O, δ¹⁵N^bulk‐N₂O, δ¹⁵N^α, δ¹⁵N^β and ¹⁵N site preference) of emitted N₂O. The maximum N₂O flux (6.9 ± 1.8 kg N ha^?1 day^?1) occurred 3 days after amendment application, followed by the maximum N₂ flux on day 4 (6.6 ± 3.0 kg N ha^?1 day^?1). The δ¹⁵N^bulk was initially ?34.4‰ and increased to +4.5‰ during the periods of maximum N₂ flux, demonstrating fractionation during N₂O reduction, and then decreased. The δ¹⁸O‐N₂O also increased, peaking with the maximum N₂ flux and remaining stable afterwards. The site preference (SP) decreased from the initial +7.5 to ?2.1‰ when the N₂O flux peaked, and then simultaneously increased with the appearance of the N₂ peak to +8.6‰ and remained stable thereafter, even when the O₂ supply was removed. We suggest that this results from a non‐homogenous distribution of NO in the soil, possibly linked to the KNO₃ amendments to the soil, causing the creation of several NO pools, which affected differently the isotopic signature of N₂O and the N₂O and N₂ fluxes during the various stages of the process. The N₂O isotopologue values reflected the temporal patterns observed in N₂O and N₂ fluxes. A concurrent increase in ¹⁵N site preference and δ¹⁸O of N₂O was found to be indicative of N₂O reduction to N₂.