ABSTRACT
Agroecosystems that support wide range of field crops and plantations were all derived or developed on soils that supported natural vegetation, forests, prairies or wasteland. The dynamics of plant species, their growth pattern and most importantly carbon and mineral nutrient dynamics in these natural habitats were actually result of wide range of interactions through the ages. Such established natural ecosystems had to be removed progressively or at times drastically to make way to large scale cropping. The nutrient dynamics that ensued in the developing agroecosystems was markedly different from those known for several decades. Conversion of natural vegetation, prairies and scrubland to cropland almost always affects carbon dynamics, rather immensely. Obviously, the inherent soil fertility, biomass production, its recycling and weather pattern that induce changes in soil carbon pools are important aspects to note. There are several studies that have aimed at understanding the effects of changes in vegetation pattern on carbon dynamics. Let us consider a few examples from the Great Plains of North America. Here, pine forests, shrub land, natural prairies and waste land have given way to large scale production of cereals like wheat, maize, sorghum, millets or cotton and plantation crops. The cropping system adopted in different regions has specific influence on carbon dynamics. Long term effect on soil-C sequestration pattern, carbon recycling and loss due to emission need attention. In the Midwestern USA, C and N declined in regions that supported maize. The decline in soil-C was greater at 38% of the original level recorded 60 years ago. In case of natural vegetation, decline of soil-C was marginally low at 32% of original [1]. Hass et al. [2] reported 39-42% decline in soil organic carbon (SOC) due to maize cultivation in the Corn Belt. Reeder et al. [3] found a 16-28% decline in SOC due to continuous cropping of cereals. Incessant cropping of Chernozems found in Russian plains induced 38-42% decline of SOC in a matter of 50 years [4]. Several reports about decline in SOC actually pertain to upper horizon of soil that holds large portion of roots system of the crops. However, there are clear suggestions, that entire soil profile that gets explored by crop roots need to be considered. We should note that fields or even large expanses of crops reach a steady state level regarding soil-C transformations, loss/gain of SOC. Once, this steady state is reached, measurements about SOC need careful interpretation. Actually, factors such as crop rotation, residue recycling and rates of C-emissions may play a vital role in stabilizing SOC of a field. Pikul et al. [5] report that monocrop of corn induced a loss of 2.3 t C/habut a rotation with corn-alfalfa-wheat resulted in only 0.3 t C/haloss. Generally, a well-diversified cropping system conserves SOC better than a mono-cropping stretch.
