Continuous cultivation does not always decrease soil organic carbon content

It is generally known that continuous cultivation causes a decline in soil organic carbon and nutrient contents. This has been shown by many years of research on upland soils starting with the classic study by Nye and Greenland (1960). Our studies in the volcanic mountain of Leyte, Philippines, have also confirmed this (e.g. Asio et al., 1998; Navarrete and Tsutsuki, 2008).

But a recent paper by Benbi and Brar (2009) published in the international journal Agronomy for Sustainable Development does not support this widely held view. In fact, they showed that intensive cultivation increased soil organic carbon by 38 % after 25 years. These researchers evaluated the impact of intensive cultivation of an irrigated and optimally fertilized rice-wheat system in Punjab, India, and found that intensive cultivation enhanced carbon sequestration due to improved crop productivity, greater belowground C transport to the soil and reduced organic matter decomposition during the wetland rice season.

Results of the study also revealed that the rice-wheat cropping in alkaline soils creates a favourable pH environment by lowering soil pH towards neutrality. During the 25-year period, the soil pH declined from 8.8. to 7.7 which resulted in the improvement in nutrient availability. Continuous application of phosphoric fertilizer led to build-up of soil P and the magnitude of accumulation was proportional to the amount of fertilizer applied.

References

Asio V.B., R Jahn, K. Stahr, and J. Margraf. 1998. In: Soils of Tropical Forest Ecosystems (A. Schulte and D. Ruhiyat, eds.). Springer Verlag, Berlin, pp: 29-36.

Benbi D.K. and J.S. Brar. 2009. A 25-year record of carbon sequestration and soil properties in intensive agriculture. Agron. Sustain. Dev. 29: 257-265.

Navarrete IA and K Tsutsuki. 2008. Land-use impact on soil carbon, nitrogen, neutral sugar composition and related properties in a degraded Ultisol in Leyte, Philippines. Soil Science and Plant Nutrition 54: 321-331.

Nye P.H. and D.J. Greenland. 1960. The soil under shifting cultivation. Commonwealth Agricultural Bureau, England.