Springer Science and Business Media LLC

The growing concern about the sustainability of tropical agricultural systems stands in striking contrast to a world-wide decline in the use of soil-improving legumes. It is timely to assess the future role that soil-improving legumes may play in agricultural systems. This paper reviews recent progress, potential, and limitations of green manure technology, using lowland rice cropping systems as the example. Only a few legume species are currently used as green manures in lowland rice. Sesbania cannabina is the most widely used pre-rice green manure for rice in the humid tropics of Africa and Asia. Astragalus sinicus is the prototype post-rice green manure species for the cool tropics. Stem-nodulating S. rostrata has been most prominent in recent research. Many green manure legumes show a high N accumulation (80–100 kg N ha-1 in 45–60 days of growth) of which the major portion (about 80%) is derived from biological N2 fixation. The average amounts of N accumulated by green manures can entirely substitute for mineral fertilizer N at current average application rates. With similar N use efficiencies, green manure N is less prone to loss mechanisms than mineral N fertilizers and may therefore contribute to long-term residual effects on soil productivity. Despite a high N2-fixing potential and positive effects on soil physical and chemical parameters, the use of green manure legumes for lowland rice production has declined dramatically world-wide over the last 30 years. Land scarcity due to increasing demographic pressure and a relatively low price of urea N are probably the main determining factors for the long-term reduction in pre-rice green manure use. Post-rice green manures were largely substituted for by high-yielding early-maturing grain legumes. Unreliability of green manure performance, non-availability of seeds, and labor intensive operations are the major agronomic constraints. The recognition and extrapolation of niches where green manures have a comparative advantage may improve an often unfavorable economic comparison of green manure with cash crop or fertilizer N. Socio-economic factors like the cost of land, labor, and mineral N fertilizer are seen to determine the cost-effectiveness and thereby farmers' adoption of sustainable pre-rice green manure technology. Hydrology and soil texture determine the agronomic competitiveness of a green manure with N fertilizers and with alternative cash crops. In general, the niches for pre-rice green manure are characterized by a relatively short time span available for green manure growth and a soil moisture regime that is unfavorable for cash crops (flood-prone rainfed lowlands with coarse-textured soils). Given the numerous agronomic and socio-economic constraints, green manure use is not seen to become a relevant feature of favourable rice-growing environments in the foreseeable future. However, in environments where soil properties and hydrology are marginal for food crop production, but which farmers may be compelled to cultivate in order to meet their subsistence food requirements, green manures may have a realistic and applicable potential.

Results from a simple model for computing long-term changes in soil organic nitrogen and crop uptake and crop recovery of nitrogen applications were compared with results of previously published long-term field trials. In these trials a rotation of sugar beet, barley, potatoes and wheat was practiced in the German Democratic Republic and continuous production of wheat and rice in the United Kingdom and Japan, respectively. The agreement between observed values for the change in soil nitrogen and nitrogen uptake and the computed values is in general satisfactory. The number of input data required for application of the model is kept as restricted as possible. Nevertheless, part of the required information may not be available in many situations. The quality of the predictions of the model will then be limited particularly by the lack of suitable input data.

Vi khuẩn đã được đếm và lượng nitơ khoáng hóa được đo trong một loại đất cát pha, một loại đất sét pha, một loại đất sét và một loại đất sét hữu cơ bằng cách ủ môi trường thạch dùng để đếm và các mẫu đất (a) trong đĩa petri ở 29°C trong môi trường thí nghiệm, (b) trong đĩa petri chôn trong đất cát pha, và (c) trong đĩa petri chôn trong đất cát pha được bổ sung một phần trăm bột cỏ lucerne. Hầu hết vi khuẩn được phát hiện trong thí nghiệm (a) và ít nhất trong thí nghiệm (c). Trong một số trường hợp, thí nghiệm (c) đã kích thích quá trình khoáng hóa nitơ. Tuy nhiên, các kết quả thu được vẫn chưa rõ ràng. Trong loại đất cát pha, đất sét pha và đất sét, ít vi khuẩn phân hủy protein được tìm thấy sau 6 tuần ở thí nghiệm (c) so với các thí nghiệm (a) hoặc (b). So với thí nghiệm (a), ít vi khuẩn phân hủy tinh bột được tìm thấy ở thí nghiệm (c) chỉ trong đất sét pha và đất sét.

Planting strategies can be effective mechanisms to reduce diffuse pollution from agricultural catchments reaching water bodies. Plants with antimicrobial properties such as mānuka (Leptospermum scoparium) demonstrated in controlled conditions the ability to inhibit nitrification and growth of pathogens in soils. This potential in a real on-farm setting was still to be investigated. In a stock-excluded riparian area, planted with mānuka on a dry stock farm, synthetic excrement patches high in urea (950 kg N ha−1 equiv.) and Escherichia coli (7.9 × 109 cfu plant-1) underneath mānuka saplings and pasture were applied. Soil was sampled at three depths over 21 days after the excrement application and analysed for total C and N, inorganic N, pH, soil moisture and E. coli. There was no significant difference between the pasture and mānuka for total C and N, C:N ratio, and soil moisture. E. coli was only different between both at 20–30 cm deep. NO3− - N and NH4+ - N concentrations were significantly lower under mānuka compared to pasture for the upper two soil depths (NO3− - N: 109 mg kg−1 vs 205 mg kg−1 in the topsoil). The results of this study indicate that mānuka may inhibit urease activity and nitrification and could reduce on-farm nitrate leaching, while also highlighting that field conditions make quantifying such phenomenon more complex.

The response of the rice cultivars ‘M9’ and ‘M-201’ to nutrient cultures salinated at −0.4 MPa with varying ratios of Na and Ca was studied. Although the dry matter production of both cultivars was sensitive to the Na/Ca ratio, this correlation was significant only for M-201. Calcium nutrition was severely affected by the composition of the external solution, and the laminae exhibited Ca-deficiency symptoms at Na/Ca molar ratios of 78 and 198. Sodium concentration in the shoot decreased as the Na/Ca ratio in the external solution decreased. Patterns of Na and Cl distribution in the shoot tissues were similar; both ions were accumulated preferentially in the tillers and older leaves. The Na-induced inhibition of Ca uptake and transport appears to be more limiting to shoot growth of M9 and M-201 than Na toxicity per se.

The exact number of replicates required for a given probability of isolating all the species of nematophagous fungi present in a single sample of soil was measured. Using the soil sprinkling technique twenty replicate sub-samples of a sample of farm soil were examined for species. Eight replicates were required under these circumstances to be 95% sure of isolating all the species present. An equation to calculate the number of replicates to achieve a given probability of 100% species isolation is also given.

A pot experiment was conducted with rice to study the relative absorption of urea in molecular form compared to the other forms of N produced in soil from the applied urea. A method involving application of 14C-labelled urea and 15N-labelled urea alternately in two splits was used to quantify the absorption of molecular urea and other forms of N formed from it. Biomass production and N uptake were greater in plants grown under flooded soil conditions than in plants grown under non-flooded (upland) conditions. Absorption of N by rice increased with increasing rate of urea application up to 250 mg pot−1 and declined thereafter. The absorption of urea from the flooded soil constituted 9.4% of total N uptake from applied N compared to only 0.2% from the non-flooded. Under submerged conditions, absorption of urea from topdressing was about twice that from basal application at planting. High water solubility of the fertilizer and better developed rice root system might have enhanced the absorption of molecular urea by flooded rice, especially from topdressing. Thus, in the flooded rice system, the direct absorption of molecular urea from topdressing accounted for 6.3% of the total N uptake from added urea. Under upland condition, it was 0.12%.