CHANGES IN SOIL CARBON INVENTORIES FOLLOWING CULTIVATION OF PREVIOUSLY UNTILLED SOILS

Cultivation of previously untilled soils usually results in release of carbon from the soil to the atmosphere, which can affect both soil fe rtility locally and the atmospheric burden of CO2 globally. Generaliza tions about the magnitude of this flux have been hampered by a lack of good quality comparative data on soil carbon stocks of cultivated and uncultivated soils. Using data from several recent studies, we have r eexamined the conclusions of previous reviews of this subject. The dat a were divided into subsets according to whether the soils were sample d by genetic horizon or by fixed depths. Sampling by fixed depths appe ars to underestimate soil C losses, but both subsets of data support e arlier conclusions that between 20% and 40% of the soil C is lost foll owing cultivation. Our best estimate is a loss of about 30% from the e ntire soil solum. Our analysis also supports the conclusion that most of the loss of soil C occurs within the first few years (even within t wo years in some cases) following initial cultivation. Our analysis do es not support an earlier conclusion that the fractional loss of soil carbon is positively correlated to the amount of carbon initially pres ent in the uncultivated soil. We found no relation between carbon cont ent of uncultivated soil and the percentage lost following cultivation .