Ths report addresses the following two questions: 
1) What are the loads (flux) of nutrients transported from the Mississippi-Atchafalaya River Basin to the Gulf of Mexico, and where do they come from within the basin? 
2) What is the relative importance of specific human activities, such as agriculture, point-source discharges, and atmospheric deposition in contributing to these loads? 
These questions were addressed by first estimating the flux of nutrients from the Mississippi-Atchafalaya River Basin and about 50 interior basins in the Mississippi River system using measured historical streamflow and water quality data. Annual nutrient inputs and outputs to each basin were estimated using data from the National Agricultural Statistics Service, National Atmospheric Deposition Program, and point-source data provided by the USEPA. Next, a nitrogen mass balance was developed using agricultural statistics, estimates of nutrient cycling in agricultural systems, and a geographic information system. Finally, multiple regression models were developed to estimate the relative contributions of the major input sources to the flux of nitrogen and phosphorus to the Gulf 
of Mexico.

Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin

Nitrogen (N) is a major fertiliser for agriculture and food production. About 67.84 million tons of N are annually applied to agricultural fields, without which nearly half of the world’s population would not be alive today. Returning plant residues to the soil is an alternative and sustainable way of N fertilisation. Although impacts of returning plant residues on plant available N in soil have been widely studied, there is still no systematic review of their mechanisms and models. In this review we highlight the following advances: (1) When plant residues are returned to the soil, N undergoes biotic immobilisation–remineralisation, abiotic immobilisation, soil organic N mineralisation and plant residue organic N mineralisation. (2) Plant residues modify inorganic N fate using three mechanism mineralisation, immobilisation–mineralisation and immobilisation, depending on plant residue nature and soil properties. (3) The use of plant residue C/N ratio is not always effective to predict the effect of plant residues. Instead, soil properties and the forms of carbon and nitrogen should be considered. (4) Mineralisation always promotes N uptake by crops and increases the risk of N loss. In addition, although net immobilisation is involved in immobilisation–mineralisation and immobilisation, it does not necessarily induce lower crop nitrogen uptake. Results also depend on the synchronism between the changing soil inorganic N and the crop N uptake. (5) N loss during mineralisation can be reduced by an immobiliser. Net N immobilisation during immobilisation–mineralisation and immobilisation can be reduced by changing the timing of ploughing and fertilising or by changing the plant residues placement.

/pdf/soil-nitrogen-dynamics-and-crop-residues-a-review-4pnjqanwqg.pdf

Soil nitrogen dynamics and crop residues. A review

The research is focused on an ecologically sound and highly productive cultivation system for fodder and/or biomass for thermal power generation on the basis of winter legumes and maize as subsequent summer crop, managed without additional nitrogen fertiliser. Therefore the yield of biomass and N-fixing capacity of a winter pea (Pisum sativum L.) and crimson clover (Trifolium incarnatum L.) monocropped and intercropped with rye (Secale cereale L.) were examined for five years in a field trial. In mid-June above-ground biomass of winter crops was removed and maize transplanted. The winter crops achieved maximum dry matter yield about three to five weeks before maturity. Mixed stands yielded more biomass than pure stands and exhibited greater yield stability. The relative advantage of intercropping, expressed as land equivalent ratio (LER), determined for intercropped winter pea/rye were 1.1 to 1.2 and for crimson clover/rye 1.3. At maturity, the amount of fixed nitrogen ranged between 178 kg N for crimson clover and 242 kg N ha-1 for winter pea, respectively. At the end of anthesis (middle of June, harvesting stage for silage fodder) 75% and 88% of the total fixed nitrogen was achieved, for clover and pea, respectively. In intercropping the amount of fixed nitrogen was lower than in pure stands due to a lower seed density of the legume; however, the N-fixing efficiency was greater than in pure stands. N-release of the winter pea in a pure stand produced a maximum yield in maize (Zea mays L.) without additional N-fertiliser. An additional N mineral fertilisation of 75 to 150 kg N and 75 to 225 kg N was necessary to achieve maximum yields in maize following intercropped winter pea and crimson clover, respectively. Legumes in mixed stands with rye resulted in lower amounts of residual nitrogen after maize harvest. The beneficial effect of legumes on maize can be divided into N-effects and rotation effects. Both effects were positive regarding winter pea. The rotation effect of crimson clover in pure stands on maize was negative. Allelopathic effects and the high sensitivity of crimson clover to mineral nitrogen in the soil, released by residues of the preceding crop, winter rape (Brassica rapa L.), were discussed as the reason for this observation. The combination of the winter pea in pure stand and maize achieved the highest total biomass yield from winter and summer crops, unfertilised (156 dt ha-1 dry). The combinations of intercropped legumes and maize produced biomass yields of 142 to 145 dt ha-1. Because winter pea is highly susceptible to lodging, intercropping with low seed density of rye is recommended (3/4 winter pea, 1/4 rye). The rye crop prevents lodging by providing support and high rates of N-fixation are achieved with high seed density of pea. Intercropping with crimson clover and rye should be based on high seed densities of legumes, too because rye is highly competitive within those mixtures.

Biomass yield and nitrogen fixation of legumes monocropped and intercropped with rye and rotation effects on a subsequent maize crop

A major charge to agricultural research is to identify management practices that maintain long-term soil fertility and crop production with reduced chemical inputs and tillage operations. Of prime importance is gaining a fundamental understanding of soil physical, chemical, and biological factors limiting crop production. In general, soil water content of the root zone is the major limitation to root growth, but many other dynamic variables also affect the root extraction of nutrients.

Root extraction of nutrients associated with long-term soil management

BOOK REVIEW By F.A. Gumbs - Terrestrial nitrogen cycles: processes, ecosystem strategies and management impacts

Almost all variables involved in a system of soil and crop management can influence N cycling and the accounting of N in the ecosystem. Consequently, research related to the effects of agriculture upon nitrate accumulation in ground water must address all aspects of both the N cycle and the hydrological cycle. Dominant in any accounting of N is the yield goal for the production enterprise. This establishes a minimum value for the total N required by the crop, but crop N requirement must be adjusted for efficiency of N uptake, climate, and for management practices. Credits must also be given for the amount of N derived from mineralization of soil organic N, manures, green manures, crop residues, or various types of wastes, plus that added through precipitation, foliar absorption of ammonia, biological N fixation, or nitrates in irrigation water. The deficit between crop N requirement and N from these sources is normally corrected by fertilization. A major problem exists in determining what is sufficient fertilizer N. One must consider the crop, weather, soil properties, and a multitude of fertilizer practices (carriers, time of application, rate, and placement). Also, many management practices can influence fertilizer N efficiency–tillage and crop residue placement, cropping system, irrigation practices, weed control, and others. Consequently, fertilizer N use efficiency can and does vary greatly. All these variables affect soil microbial activity and N immobilization-mineralization rates. Leaving crop residues on the soil surface, for example, creates a cooler and wetter environment than in bare soils, affecting activities of soil microorganisms and resulting in differential effects on N cycling and availability. Quantitative effects of a practice are often site-specific, and can only be predicted through process-based computer simulation models. Until such models are developed, the best approach is to fertilize according to field-calibrated soil tests. Although an empirical approach, when restricted to the area from which the field calibration was derived, this approach can provide reasonable estimates of minimum fertilizer N requirements. Previous research has shown that fertilizer use based on such recommendations reduces production costs and nitrates leaching, while maintaining yields. With continued research, well calibrated soil test recommendations for fertilizer usage can provide the accounting of N needed to minimize nitrate leaching into ground water.

Proper accounting for N in cropping systems.

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Proper accounting for N in cropping systems.