Global Theme on Agroecosystems

Abstract

Soil organic carbon (SOC) plays an important role as a source of plant nutrients and in maintaining the soil integrity. Any land use management that increases SOC by removing CO2 from the atmosphere by storing it in the soil, is termed as carbon sequestration. This study was conducted to learn about the role of various agricultural practices on soil nutrient dynamics and its relationship with SOC in various land use systems of semi-arid tropics (SAT). The study area covered 28 SAT benchmark spots, 21 out of which were on black soils and 7 on red soils, covering areas of 15.29 m ha and 6.34 m ha, respectively. Soils were sampled from the benchmark sites/pedons during 2000–03 and processed for chemical analysis. It was observed that irrespective of bioclimatic zones, land use under horticultural and agricultural systems in general, and paddy systems in particular, had maximum content of organic carbon and total N. The soil parameter viz. clay fraction also influenced the total N and total P, and hence organic carbon in black and red soils showed significant positive correlation with total N and P. Results indicated that perennials could sequester carbon better when compared to annual crops. The nutrient stocks and soil organic C and N ratio (carbon/nitrogen C:N), and carbon/phosphorus (C:P) were computed in addition to SOC for the purpose of identifying the maintained soil quality. It was observed that the C:N ratio varied from 16:1 to 22:1 under different zones and it was highest under semi-arid (moist) zones in black soils. Similarly C:P ratio of soils under various bioclimatic zones revealed that it was highest under sub-humid (moist), followed by arid zone and lowest under semiarid zones. The C:N ratio of studied soils under various systems was wider than commonly accepted values reported for other tropical soils. The mean total N content of black soils was 0.042% and in case of red soils it was 0.052%, which corresponds to a minimum threshold level of 0.063% and 0.078% for black and red soils, respectively. Thus within the defined range of C:N ratios, those soils having SOC content of above values was considered along with minimum threshold values of total nitrogen stocks (Mg ha-1) to arrive at the better systems. The minimum values of TN stocks was calculated with the established equation and the values for the corresponding levels of SOC was found to be 1.95 Mg ha-1 for black soils and 2.30 Mg ha-1 for red soils (both the soils types having an average bulk density of 1.5 Mg m-3). Thus the soil total N stocks of systems that were found above the minimum threshold values are considered as better production systems. The nutrient stocks and nutrient ratio in addition to soil organic carbon was used as the main criteria to develop the soil C:N index. The index varied between 0.27 and 0.87 with an average of 0.57 under the various systems spread over different bioclimatic zones and soil types. The variation of soil C:N index in different soil types showed that, the fertility status of red soils in terms of SOC and soil nutrient stocks in majority of the pedons was higher as compared to black soils. The variation in the soil C: N index due to bioclimatic zones in black and red soils, showed that semiarid (moist) zone in black soils had the highest soil C: N index while the lowest was observed in sub-humid (moist) zone. As the MAR decreased from 1200 mm to 850 mm, the index increased from 0.30 to 0.38. Thus among the zones, the semi-arid moist was found to sequester more carbon. The variation in soil C:N index in different land use based systems such as horticultural (0.50) and forest systems (0.40) had better C:N index as compared to agricultural system in black soils. In red soils, forest system (0.76) had better C:N index as compared to agricultural system dominated by annual crops. Another significant observation was that permanent fallow land also had the potential to sequester carbon based on the magnitude of soil C:N index. The variation in the soil C:N index with the three major crop based systems studied showed that cereal based cropping systems sequester more carbon as compared to cotton and soybean based systems and can be promoted.