Since the early 1990s about 1/4 part of the arable soil area of Russian Federation have been abandoned due to the economic depression (Pankova and Novikova, 2000). When cultivated lands are abandoned, they usually accumulate carbon and nitrogen, both in vegetation and soil (Poulton et al., 2003). The average rates of carbon accumulation in the soils are similar after land use changes from cropland to grassland or forest and amount 33-34 g C m^-2 y^-1 (Post and Kwon, 2000). Changes in farming management alter the quality of organic matter in the soil (Nilsson et al, 2004) and the deepness of these alterations will be caused by the duration of abandonment. The objects of this study were: a) to determine the microbial activity of the abandoned lands of different ages; b) quantify the changes of total, dissoluble and microbial C-pools in the soil after different periods of abandonment and c) to estimate the rate of carbon accumulation.

 The investigations were carried out on the arable soil under winter wheat and soils that were abandoned 2, 6, 11 and 26 years ago after land use change from crops to permanent grassland (loamy Phaeozems, Moscow region, Russia; 54^o50'N, 37^o35'E). There are three kinds of management in the 11-yrs abandoned lands: cut, uncut and reforestation. The total soil organic carbon (SOC) was determined to the depth of one meter. Dissolved organic carbon (DOC, dichromate oxidation method), carbon immobilized in microbial biomass (C_mic, SIR method) and microbial (respiratory) activity were determined in the 0-60 cm layer.

It was found that microbial activity (MA) of the soils studied comprised 0.24-0.99 μg C-CO₂·g^-1·h^-1 in the topsoil (0-10 cm) and 0.01-0.12 μg C-CO₂·g^-1·h^-1 in the deeper soil layers (30-60 cm). The microbial activity was lowest in the arable soil (0.27-0.33 μg C-CO₂·g^-1·h^-1) and highest in the 26-yrs grassland (0.77-1.35 μg C-CO₂·g^-1·h^-1). As a rule, MA increased proportionately to the age of abandonment. The amount of SOC of the soils studied was the highest in the 0-5 cm layer and decreased sharply with depth. The SOC in 0-5 cm layer of arable soil and abandoned lands of 2, 6 and 11 yrs (uncut and reforestation) were very close and averaged 1.18±0.04 %, while the SOC-content of 11-yrs (cut) and 26-yrs (uncut) grasslands were significantly higher (1.99±0.13%). Carbon stores in the soils studied varied from 1.55 to 2.12 kg C m^-2  in the 0-10 cm layer and from 5.08 to 7.50 kg C m^-2in the 0-100 cm layer. The accumulation rates SOC in the former plough-layer (0-20 cm) caused by the conversion from cultivation to permanent grassland have been found to be 30-81 g C m^-2 y^-1. Minimal DOC values were obtained in the arable soils: 2.8-4.7 mg C×100 g^-1 of soil. Soils under 26-yrs grassland contained the highest amount of dissolved and microbial carbon: DOC=8,1-10,2 mg C·100 g^-1 of soil and C_mic=30-88 mg C·100 g^-1 of soil. As a rule, amounts of DOC and C_mic (0-20 cm) increased proportionately to the period of conversion to grassland. There was a great difference between the amount of microbial carbon in topsoil and in the deeper levels of soil. The former plough layer contains about 5-10 times more microbial carbon than the deeper levels of soil. It has been show that the management of the former agricultural lands influenced the total and microbial C-pools in 0-5cm layer. The amounts of SOC and C_mic in the cut variant of the 11-yrs abandoned land were significantly higher than in uncut and reforestation variants.

Therefore, the conversion of arable soils to permanent grasslands lead to increase of microbial activity, dissolved, and microbial carbon pools in the topsoil, carbon sequestration in the soil organic matter.  

 This study was supported by Russian Foundation for Basic Researches and Program of Presidium of RAS N 13.

References

Nilsson K.S., Hyvonen R, Agren G.I. (2004), Using the continuous-quality theory to predict microbial biomass and soil organic carbon following organic amendments. Eur Journ Soil Sci., 1-9

Pankova, E.I., and A.F. Novikova (2000), Soil degradation processes on agricultural lands of Russia, Eurasian soil Sci., 33: 319-330

Paustian, K., J. Six, E. T. Elliott, and H. W. Hunt (2000), Management options for reducing CO₂ emissions from agricultural soils, Biogeochemistry, 48: 147-163.

Poulton, P. R., E. Pye, P. R. Hargreaves, and D. S. Jenkinson (2003), Accumulation of carbon and nitrogen by old arable land reverting to woodland, Global Change Biology, 9: 942-955.