Net Primary Production of Steppe Ecosystems and the Reasons Underlying Its Spatial Variation

Abstract

The changes in net primary production—aboveground (ANP), belowground (BNP), and total (NPP)—are considered for meadow, true, and dry steppes. The investigated meadow and true steppes are found between 36° and 116° E, 47° and 56° N. In Tyva, the production of dry steppes has been determined for different landscape positions: from the mountaintop to the bottom of intermontane depression. The ANP value in meadow steppes changes eastward from 10.2 to 3.1 t/ha per year; in true steppes, from 5.8 to 0.7 t/ha per year and depends on many factors, such as air temperature, precipitation, and soil properties. The latter are controlled by a set of factors, including the topographic position of ecosystem, which determines different soil moistening. The general trend of the eastward decrease in the ANP value is often disturbed both in meadow steppes and true steppes. In some cases, ANP increases rather than decreases in a series of meadow steppes, which is explained by certain changes in soil conditions. The first increase in ANP from 4.8 (63° E) to 6.1 (73° E) t/ha per year occurs with the replacement of Luvic Chernozem (Loamic) by Inclinigleyic Chernozem (Loamic) resulting from additional soil moistening. The second increase from 3.6 (75° E) to 6.6 (90° E) t/ha per year is related to the change from Tonguic Chernozem (Siltic) to Haplic Chernozem (Siltic, Pachic). Three increases in ANP are observed in true steppes, namely, when (1) Skeletic Kastanozem (Siltic) is replaced by Calcic Chernozem (Siltic); (2) Haplic Solonetz (Loamic), by Calcic Chernozem (Loamic); and (3) Mollic Leptosol (Siltic), by Calcic Chernozem (Siltic). The BNP value in the upper 30‑cm-thick soil layer of meadow and true steppes generally decreases eastward from 26.8 to 7.7 t/ha per year without any evident regular pattern. In Tyva with its different relief, the ANP of dry steppes varies from 3.7 to 1.7 t/ha per year and BNP, from 27.0 to 8.7 t/ha per year. Consequently, not only air temperature and precipitation determine the ANP value in grass ecosystems but also the soil properties, such as soil structure, C_org content, nutrients, and water availability.