Is "Summer Fallowing" Beneficial to Sustainable Grain Production in Central Asia?.
Takashi Kosaki1, Shinya Funakawa2, Elmira Saljnikov3, Kanat K. Akshalov4, and Yusuke Takata2. (1) Graduate School of Global Environmental Studies, Kyoto Univ, Kyoto, 606-8501, Japan, (2) Graduate School of Agriculture, Kyoto Univ, Kyoto, 606-8502, Japan, (3) Institute of Soil Science, Belgrade, 11000, Serbia and Montenegro, (4) Barayeb Kazakh Research and Production Center of Grain Farming, Shortandy, Akmolinskaya, Kazakhstan
Summer fallowing has been commonly practiced in wheat-based rotation systems in dryland of Central Asia, in order to store water, to control weed hazard, and to accumulate mineral N through mineralization of soil organic matter. However, since summer fallowing prevents any vegetative growth by shallow tillage, it has often been reported that this practice on Chernozem soils or Ustolls had accelerated organic matter decomposition causing the risk in lowering the sustainability of agricultural production. We studied whether summer fallowing is beneficial to sustainable grain production in dryland of Central Asia from the viewpoints of soil moisture and organic matter dynamics and soil quality change.The experiment was conducted in 1998-2004 at Barayeb Kazakh Research and Production Center of Grain Farming in northern Kazakhstan, where soils are Southern Chernozem or Haplustolls with mean annual temperature of 1.6 °C and total annual precipitation of 324 mm, with the treatments of a variety of the frequencies in summer fallowing and other water-harvesting management techniques, i.e. snow collection and subsoil cutting. Soil moisture and temperature were monitored with TDR and heat probes together with data loggers, CO2 emission due to the decomposition of soil organic matter was measured in situ with alkali absorption method in a closed chamber, and plant biomass was recorded for wheat and weeds. As for the evaluation of soil quality change, potentially mineralizable C (PMC) and N (PMN) during 70 days incubation, soil mineral N (min-N) and light (SG<1.8 g cm-3) fraction organic matter (LF-C and LF-N) were analyzed besides routine physicochemical characterization of soils.Soil moisture and temperature monitoring revealed that both summer fallowing and snow collection practices accumulated additional ca. 100 mm moisture (0-90 cm soil) when practiced individually.When together, benefit of 100 mm additional moisture from snow collection was, however, cancelled out by summer fallowing, which induced subsoil freezing in spring and caused poor percolation and subsequent loss of thawed snow through surface runoff and evaporation. Snow collection with subsoil cutting is more important than summer fallowing for capturing additional moisture. Soil organic matter (SOM) dynamics in a cropping season exhibited that the output as CO2 emission modeled with Arrhenius relationship between the daily soil respiration rate and its determining factors, i.e. the soil temperature, the soil moisture, and the activation energy, was 2.5-3.0 Mg C ha-1 and the input as plant residues ranged from 0 (fallowing) to 2.5 (cropping wheat). Thus, the budget of SOM was -2.9 and -0.2 Mg C ha-1 under fallowing and cropping, respectively. SOM content is more or less stable under wheat cropping, but decreases under summer fallowing, resulting in the gradual decrease during a conventional rotation for 5 years by 2.9 Mg C ha-1, which may be equivalent to 4% of the total SOM stock in the plow layer (30 cm).As for the soil quality change, frequent summer fallowing induced the decrease in soil organic C, total N, PMC, and LF-C, and the decrease was more obvious in PMC and LF-C than in soil organic C and total N. PMN, min-N, and LF-N were less affected by the frequency of summer fallowing, but more by fallowing phase, that is, pre-fallowing phase gave significantly higher PMN and LF-N and lower min-N than post-fallowing phase. SOM degradation due to summer fallowing can be detected with the soil characteristics related to soil C status, particularly those affecting microbial activity, i.e. PMC and LF-C. During the summer fallowing, soil N fertility increases through transformation of PMN into min-N, which is, however, subject to surface runoff and/or leaching at the snow thawing period in spring before taken up by crops. We conclude that an extensive and uniform application of summer fallowing is not beneficial to sustainable grain production in Central Asia due to the above reasons, and thus, an alternative soil and land management technology must be developed with taking account of snow-collection-based water harvest management and its site-specific application in accordance with soils and topographical conditions.