A 25-year record of carbon sequestration and soil properties in intensive agriculture

As a major carbon pool on earth, soil organic carbon may act either as a sink or a source of atmospheric CO2, a greenhouse gas. Soil organic carbon is also impacting fertility, and, in turn, crop yields. However, knowledge of the impact of cropping techniques on the long-term behavior of soil carbon is scarce. Several studies have shown that continuous cropping decreases soil organic carbon stocks, rapidly in the initial years then at a slower rate, approaching a new equilibrium after 30 to 50 years. For instance, a study of intensive corn cropping for 35 years on temperate soils showed a 50% decrease in soil organic carbon. Our study is located in the North Indian state of Punjab. It is the most intensively cultivated region in the country with a cropping intensity of 190%, predominantly of a rice-wheat system. Due to high nutrient demand and its continuous cultivation, the cropping system is presumed to adversely affect soil organic carbon and other soil properties. However, this has been postulated without any real-time data analysis on a regional scale. Therefore, we evaluated soil data for 25 years from 1981/82 to 2005/06 to investigate the impact of intensive agriculture on C sequestration and soil properties on a regional scale. The results showed that, unexpectedly, intensive agriculture has resulted in improved soil organic carbon (SOC) status. As a weighted average for the whole state, SOC increased from 2.9 g kg-1 in 1981/82 to 4.0 g kg-1 in 2005/06, an increase of 38%. Increased productivity of rice and wheat resulted in enhanced C sequestration in the plough layer by 0.8 t C ha-1 per ton of increased grain production. Soil pH declined by 0.8 pH units from 8.5 in 1981/82 to 7.7 in 2005/06. This pH decline has positive implications for availability of phosphorus and micronutrients such as Zn, Fe and Mn. Changes in plant-available P in soil were related to the amount of fertilizer P applied. The status of available P in soils increased from 19.9 kg ha-1 in 1981/82 to 29.2 kg P ha-1 during 2005/06. The status of plant-available K in soil remained almost unaltered and averaged 106 and 123 mg kg-1 soil in 1981/82 and 2005/06, respectively. The analysis showed that intensive cultivation of a rice-wheat system unexpectedly resulted in improved C sequestration, a favorable pH environment and amelioration of the soil salinity

Effect of long-term fertilization in maize-wheat cropping system on carbon mineralization in soil

Quantitative information on mineralization of soil organic carbon (SOC) under different long-term nutrient management practices is essential for better assessment of carbon (C) loss from soil. With an aim to evaluate long-term nutrient management effects on C mineralization kinetics in soils, a laboratory incubation study was conducted with soils collected from different depths (0–15, 15–30, 30–60, and 60–100 cm) of a 46-year old maize-wheat experiment. The treatments in the field involved long-term application of 100% N, 100% NP, 100% NPK, 150% NPK, 100% NPK + FYM (farmyard manure), and an unfertilized control. Long-term application of fertilizers significantly (p < 0.05) influenced organic C concentration and C mineralization kinetics in soil. Integrated application of inorganic fertilizers and organic manure (100% NPK + FYM) resulted in greater SOC accumulation and higher potentially mineralizable C in soil compared to other treatments. The microbial biomass carbon (MBC), basal soil respiration (BSR), carbon mineralization (Cmin), and microbial quotient (qMic) values were significantly higher under integrated application of NPK and FYM in maize-wheat system. The results showed that long-term application of FYM along with inorganic fertilizer enhanced SOC pool compared to solitary application of mineral fertilizers