Christine A. Watson

The fate of nitrogen from incorporated cover crop and green manure residues

Nitrogen retention and release following the incorporation of cover crops and green manures were examined in field trials in NE Scotland. These treatments reduced the amounts of nitrate-N by between 10–20 kg ha-1 thereby lowering the potential for leaching and gaseous N losses. However, uptake of N by overwintering crops was low, reflecting the short day-lengths and low soil temperatures associated with this part of Britain. Vegetation that had regenerated naturally was as effective as sown cover crops at taking up N over winter and in returning N to the soil for the following crop. Incorporation of residues generally resulted in lower mineralisation rates and reduced N2O emissions than the cultivation of bare ground, indicating a temporary immobilisation of soil N following incorporation. Emissions from incorporated cover crops ranged from 23–44 g N2O-N ha-1 over 19 days, compared with 61 g N2O-N ha-1 emitted from bare ground. Emissions from incorporated green manures ranged from 409–580 g N2O-N ha-1 over 53 days with 462 g N2O-N ha-1 emitted from bare ground. Significant positive correlations between N2O and soil NO3- after incorporation (r=0.8–0.9; P<0.001 and r=0.1–0.4; P<0.05 for cover crops and green manures, respectively) suggest that this N2O was mainly produced during nitrification. There was no significant effect of either cover cropping or green manuring on the N content or yield of the subsequent oats crop, suggesting that N was not sufficiently limiting in this soil for any benefits to become apparent immediately. However, benefits of increased sustainability as a result of increased organic matter concentrations may be seen in long-term organic rotations, and such systems warrant investigation.

Models of biological nitrogen fixation of legumes. A review

Leguminous crops have the ability to fix nitrogen (N) biologically from the atmosphere. This can benefit not only the legumes themselves but also any intercropped or subsequent crops, thus reducing or removing the need to apply N fertilizers. Improved quantification of legume biological nitrogen fixation (BNF) will provide better guidance for farmers on managing N to optimise productivity and reduce harmful losses to the environment. There are many techniques available for the direct quantitative measurement of legume BNF in the field and in controlled environments. However, these are time-consuming and therefore expensive, and generate data relevant only to the time and place of measurement. Alternatively, legume BNF can be estimated by either empirical models or dynamic mechanistic simulation models. Comparatively, simulation by a dynamic model is preferable for quantifying legume BNF, because of its capability to simulate the response of N fixation to a wide range of environmental variables and legume growth status. Currently there is no published review of the approaches used to simulate, rather than measure, legume BNF. This review of peer-reviewed literature shows that most simulation models estimate the N fixation rate from a pre-defined potential N fixation rate, adjusted by the response functions of soil temperature, soil/plant water status, soil/plant N concentration, plant carbon (C) supply and crop growth stage. Here, we highlight and compare the methods used to estimate the potential N fixation rate, and the response functions to simulate legume BNF, in nine widely-cited models over the last 30 years.We then assess their relative strengths in simulating legume BNF with varying biotic and abiotic factors, and identify the discrepancies between experimental findings and simulations. After this comparison, we identify the areas where there is the potential to improve legume BNF simulation in the future. These include; (1) consideration of photosynthetic C supply, (2) refining the various effects of soil mineral N concentration, (3) characterization and incorporation of excess soil water stress and other factors into models, and (4) incorporation of the effects of grazing, coexistence and competition with intercrops and weeds into models to improve their practical relevance to sustainable agricultural systems. This review clarifies, for the first time, the current progress in legume BNF quantification in simulation models, and provides guidance for their further development, combining fundamental experimental and modelling work.

Functional aspects of root architecture and mycorrhizal inoculation with respect to nutrient uptake capacity

The aim of this research was to investigate the effect of arbuscular mycorrhizal (AM) colonisation on root morphology and nitrogen uptake capacity of carob ( Ceratonia siliqua L.) under high and low nutrient conditions. The experimental design was a factorial arrangement of presence/absence of mycorrhizal fungus inoculation ( Glomus intraradices) and high/low nutrient status. Percent AM colonisation, nitrate and ammonium uptake capacity, and nitrogen and phosphorus contents were determined in 3-month-old seedlings. Grayscale and colour images were used to study root morphology and topology, and to assess the relation between root pigmentation and physiological activities. AM colonisation lead to a higher allocation of biomass to white and yellow parts of the root. Inorganic nitrogen uptake capacity per unit root length and nitrogen content were greatest in AM colonised plants grown under low nutrient conditions. A better match was found between plant nitrogen content and biomass accumulation, than between plant phosphorus content and biomass accumulation. It is suggested that the increase in nutrient uptake capacity of AM colonised roots is dependent both on changes in root morphology and physiological uptake potential. This study contributes to an understanding of the role of AM fungi and root morphology in plant nutrient uptake and shows that AM colonisation improves the nitrogen nutrition of plants, mainly when growing at low levels of nutrients.

Plant Nutrients in Organic Farming

Effective nutrient management is essential in organic farming systems. Processed soluble fertilisers such as ammonium nitrate, which feed the plant directly and are thought to bypass the natural processes of the soil, are not generally acceptable. Nutrient supply to crop plants is supported through recycling, the management of biologically-related processes such as nitrogen fixation by clover and other legumes, and the limited use of unrefined, slowly-soluble off-farm materials that decompose in the same way as soil minerals or organic matter. The aim is to achieve as far as possible a closed nutrient cycle on the farm and to minimise adverse environmental impact. Effective management of any ‘waste’ materials such as manures and crop residues is a key to nutrient cycling on organic farms. However, not all organic farms have easy access to manures and recycling is limited by the prohibition of the use of sewage sludge because of current concerns over the introduction of potentially toxic elements, organic pollutants and disease transmission. In addition, the current global market, in which food is transported large distances from the farm, results in a significant export of nutrients. Exported nutrients must be replaced to avoid nutrient depletion of soils. Nutrient budgeting suggests some cause for concern over the sustainability of organic systems because of their dependence on feedstuffs and bedding for inputs of phosphorus (P) and potassium (K), and on the very variable fixation by legumes or imports of manure or compost for nitrogen (N); air pollution and net mineralisation from soil reserves appear to comprise a large part of the N supply on some organic farms. Losses of N from organic systems can also be as large as those from conventional systems and, being dependent on cultivation and the weather, they are even more difficult to control than those from fertilisers applied to conventional farms. There is some evidence of P deficiency in soils under organic production, and replacing K sold off the farm in produce is especially difficult. Organic farming systems may be sustainable and have the potential to deliver significant environmental benefits, but these depend on specific cropping and management practices on each farm. It is important that we study and improve nutrient management on all farm systems and in the context of plant, animal and human health in order to develop more sustainable farming systems.

Soil Phosphorus Management in Organic Cropping Systems: From Current Practices to Avenues for a More Efficient Use of P Resources

Phosphorus (P) is a major nutrient for all living organisms and a key production factor in agriculture. In crop production, it is usually supplied to soils through fertilisers or recycled manure and compost. Organic production guidelines ban the use of highly soluble, manufactured P fertilisers and, thus, recommend recycling P from livestock manure and compost. In this chapter, after an overview of P dynamics in soils, we explore the consequences of such guidelines in terms of field- and farm-gate P budget, soil P availability and crop productivity. Moreover, we propose some avenues for the more effective use of P resources, ranging from rhizosphere-based processes (e.g., soil microorganism manipulation), genotype selection and cropping practices (e.g., intercropping), to farming system design (e.g., a combination of crops and animals at the farm scale). Finally, the potential benefits of these options are compared with respect to soil P status, field- and farm-P budgets.

Revisiting the Multiple Benefits of Historical Crop Rotations within Contemporary UK Agricultural Systems

On many farms, current cultivation and planting practices hold little resemblance to those of the past. However, with volatility in fertiliser price, changing climatic conditions, stricter environmental legislation and a requirement for alternatives to high input farming, it is perhaps timely to reconsider the potential for crop rotation. Historical records show that practiced rotations have changed due to adaptation of cropping systems, machinery, inputs and economics. Despite this regional differences in rotations, in terms of length of cropping sequence, practiced in the United Kingdom, and in particular Scotland still exist and persist in many organically managed systems. Knowledge gained from past experiences can be utilised and where appropriate used to support modern cropping systems.

Improving Bioavailability of Phosphate Rock for Organic Farming

The sustainable use of nutrients in agricultural food production represents a major emphasis for international research, and evidence that clearly demonstrates the imbalance between nutrient inputs and outputs exists. Nutrient surpluses exist and are most commonly associated with intensive livestock production and present a particular range of environmentally related issues. Nutrient deficiency can also develop, and organically managed systems highlight the difficulties that are involved in maintaining agronomically acceptable concentrations of soil phosphorus (P). A restricted range of P-containing sources, often having poor solubility, exacerbate these difficulties, and obvious benefits would arise if the availability could be “naturally” enhanced. Slow rates of phosphate rock (PR) solubilization under prevailing soil conditions reduce the general agronomic usefulness and potential benefits that any direct applications might provide. Being able to improve rates of dissolution through some control of the solubilization process would offer widespread potential advantages, particularly with respect to better matching patterns of P supply with crop demand. A variety of pre and postapplication opportunities exist to improve the solubility of rock phosphate. Some of these have particular relevance to organic agriculture where phosphate rock represents an important and acceptable “external” source of P. A range of post-application, farm management practices that include green manures and rotations using crops with favorable traits that improve P utilization have been successfully employed. Here, we emphazise pre-application techniques, especially the co-composting of phosphate rock with various organic by-product materials that include livestock manures and residual vegetable matter. A range of laboratory incubations have demonstrated the underlying mechanisms involved with solubilization. The significance of microbially induced production of organic acids and acidity during composting is particularly important in this respect. While co-composting with phosphate rock offers a great potential that could be developed for use at the individual farm scale, the key controlling factors and underlying mechanisms are far from being fully understood. A possible time sequence of reactions that might be envisaged include an initial production of protons and organic acids leading to the mineralogical dissolution and release of Ca and P, followed finally by an extended period during aging of the compost where secondary reactions appear to influence the form of P. The consequences of composting conditions and individual processes on immediate and longer-term bioavailability of P once field applied are still poorly defined.