Soil Chemical Analysis

The effects of land use change on soil carbon stocks are of concern in the context of international policy agendas on greenhouse gas emissions mitigation. This paper reviews the literature for the influence of land use changes on soil C stocks and reports the results of a meta analysis of these data from 74 publications. The meta analysis indicates that soil C stocks decline after land use changes from pasture to plantation (−10%), native forest to plantation (−13%), native forest to crop (−42%), and pasture to crop (−59%). Soil C stocks increase after land use changes from native forest to pasture (+ 8%), crop to pasture (+ 19%), crop to plantation (+ 18%), and crop to secondary forest (+ 53%). Wherever one of the land use changes decreased soil C, the reverse process usually increased soil carbon and vice versa. As the quantity of available data is not large and the methodologies used are diverse, the conclusions drawn must be regarded as working hypotheses from which to design future targeted investigations that broaden the database. Within some land use changes there were, however, sufficient examples to explore the role of other factors contributing to the above conclusions. One outcome of the meta analysis, especially worthy of further investigation in the context of carbon sink strategies for greenhouse gas mitigation, is that broadleaf tree plantations placed onto prior native forest or pastures did not affect soil C stocks whereas pine plantations reduced soil C stocks by 12–15%.

Soil carbon stocks and land use change: a meta analysis

Microbial ureases: significance, regulation, and molecular characterization.

The principal objections to the proposition that organic agriculture can contribute significantly to the global food supply are low yields and insufficient quantities of organically acceptable fertilizers. We evaluated the universality of both claims. For the first claim, we compared yields of organic versus conventional or low-intensive food production for a global dataset of 293 examples and estimated the average yield ratio (organic:non-organic) of different food categories for the developed and the developing world. For most food categories, the average yield ratio was slightly 1.0 for studies in the developing world. With the average yield ratios, we modeled the global food supply that could be grown organically on the current agricultural land base. Model estimates indicate that organic methods could produce enough food on a global per capita basis to sustain the current human population, and potentially an even larger population, without increasing the agricultural land base. We also evaluated the amount of nitrogen potentially available from fixation by leguminous cover crops used as fertilizer. Data from temperate and tropical agroecosystems suggest that leguminous cover crops could fix enough nitrogen to replace the amount of synthetic fertilizer currently in use. These results indicate that organic agriculture has the potential to contribute quite substantially to the global food supply, while reducing the detrimental environmental impacts of conventional agriculture. Evaluation and review of this paper have raised important issues about crop rotations under organic versus conventional agriculture and the reliability of grey-literature sources. An ongoing dialogue on these subjects can be found in the Forum editorial of this issue.

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Organic agriculture and the global food supply

The effects of irrigation-induced salinity and sodicity on the size and activity of the soil microbial biomass in vertic soils on a Zimbabwean sugar estate were investigated. Furrow-irrigated fields were selected which had a gradient of salinity and sugarcane yield ranging from good cane growth at the upper ends to dead and dying cane at the lower ends. Soils were sampled under dead and dying cane, poor, satisfactory and good cane growth and from adjacent undisturbed sites under native vegetation. Electrical conductivity (EC) and sodium adsorption ratio (SAR) of saturation paste extracts was measured, as well as the exchangeable sodium percentage (ESP). There was a significant negative exponential relationship between EC and microbial biomass C, the percentage of organic C present as microbial biomass C, indices of microbial activity (arginine ammonification and fluorescein diacetate hydrolysis rates) and the activities of the exocellular enzymes beta-glucosidase, alkaline phosphatase and arylsulphatase but the negative relationships with SAR and ESP were best described by linear functions. By contrast, the metabolic quotient increased with increasing salinity and sodicity, exponentially with EC and linearly with SAR and ESP. Potentially mineralizable N, measured by aerobic incubation, was also negatively correlated with EC, SAR and ESP. These results indicate that increasing salinity and sodicity resulted in a progressively smaller, more stressed microbial community which was less metabolically efficient. The exponential relationships with EC demonstrate the highly detrimental effect that small increases in salinity had on the microbial community. It is concluded that agriculture-induced salinity and sodicity not only influences the chemical and physical characteristics of soils but also greatly affects soil microbial and biochemical properties. (C) 2003 Elsevier Science Ltd. All rights reserved.

Effects of irrigation-induced salinity and sodicity on soil microbial activity

The mineralization of N in salt-affected soils is a subject of much controversy because there are reports of salt induced non-biological ammonification. In laboratory experiments we studied the decomposition of green manure Sesbania in saline and alkali (sodic) soils. Carbon mineralization was reduced from 38.6 to 16.8% when electrical conductivity (ECe) in saline soils was 97 dS m −1 . In alkali soils C mineralization was ∼38.0% irrespective of pH variation from 8.1 to 10.0. Ammoniacal-N accumulated up to ECe 70 dS m −1 beyond which it declined; nitrite+nitrate-N declined at ECe 16 dS m −1 and was not detectable at ECe≥26 dS m −1 . Net N mineralized decreased from 351.2 mg kg −1 in the non-saline control soil (ECe 1.1 dS m −1 ) to 277 mg kg −1 at ECe 97 dS m −1 salinity and from 399.2 in control (pH 8.1) to 317 mg kg −1 at pH 10.0 in alkali soils. There was no NH 3 volatilization in the saline soils and it was negligible in the alkali soils; most of the N lost was presumably due to denitrification. Protease, amidase (asparaginase, glutaminase) and deaminase (histidase) activities in soil were low but were stimulated by organic matter addition. Glutaminase activity was appreciable at high pH and deaminase at high salinity. The depression of ammonification at ECe 97 dS m −1 and the overall decrease in net N mineralization with increasing pH or salinity, and the significant enzymatic hydrolysis of organic N at high pH or salinity were all evidence of the biological nature of N mineralization in salt-affected soils. The adaptation of nitrifiers to salt stress at later stages of incubation, the nature of N losses and the reduction in losses with increase in pH or salinity also indirectly showed that N mineralization was biological and not chemical.

Carbon and nitrogen mineralization from added organic matter in saline and alkali soils

The effects of salinity and sodicity upon nodulation and nitrogen fixation in chickpea (Cicer arietinum).

Salt‐affected soils have low biological activity both because of osmotic and ionic effects of salts and owing to iimitation of carbonaceous substrates. Alkali soils and artificially salinized soils were incubated at 28°C for 3 months with or without organic (1% Sesbania cannabina green manure) amendment, at 60% water‐holding capacity. CO2 evolution was measured by absorption in alkali, microbial biomass by chloroform fumigation‐incubation, dehydrogenase activity by tetrazolium reduction, and urease activity by measuring ammonia release titrimetrically. CO2 evolution and microbial biomass were reduced at pH 8.7 in unamended alkali soils by 18% and 35%, respectively; at pH 10.3 the reduction was by 83% and 35%, respectively. Organic matter (OM) amendment stimulated CO2 evolution, and the biological activity in soils up to pH 10.0 was restored to that of amended normal soil (pH 8.1). The activity of pH 10.3 soil was 78% of normal. Extent of added carbon decomposed was ∼38% throughout the pH range studied. CO...

Ameliorative influence of organic matter on biological activity of salt‐affected soils

Potential of wastelands for sequestering carbon by reforestation

The influence of surface growth of inoculated cyanobacteria (blue-green algae) on subsurface properties of a brown earth, silt loam soil was studied in reconstituted flooded soil columns. One blue-green algae species, Nostoc muscorum, become dominant within the first 7 days of inoculation. In light control columns (not inoculated) a bryophyte, Barbula recurvirostra, was dominant although significant growth of indigenous blue-green algae occurred. The blue-green algae counts were in the range of 1×106 g-1 dry soil in the surface layer (0–0.7 cm) in both columns. Any effect of surface phototrophic growth on soil properties was restricted to the surface layer. In inoculated columns there was a twofold increase in microbial biomass and an eightfold increase in bacterial numbers by week 13. However, bacterial numbers declined so that there was only a 2.8-fold increase by week 21. Dehydrogenase (x2.1), urease (x2.8) and phosphatase (x3.1) activities and polysaccharides (+69%) increased by week 21 as a result of the blue-green algae inoculation along with a significant improvement in soil aggregation. However, similar increases occurred in the light control columns, indicating that given appropriate conditions of light and moisture indigenous species may be ultimately as effective as introduced species in bringing about biochemical and microbiological changes to soil.

D. L. N. Rao

Papers

Carbon and nitrogen mineralization from added organic matter in saline and alkali soils

The effects of salinity and sodicity upon nodulation and nitrogen fixation in chickpea (Cicer arietinum).

Ameliorative influence of organic matter on biological activity of salt‐affected soils

Potential of wastelands for sequestering carbon by reforestation

The effect of surface growth of blue-green algae and bryophytes on some microbiological, biochemical, and physical soil properties