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