Net Primary Production of Steppe Ecosystems and the Reasons for its Spatial Variability

The purpose of the article is to analyze the values change of net primary production: aboveground (ANP), belowground (BNP) and total (NPP) for meadow, true and dry steppes. The investigated meadow and true steppes are located between 36 and 116° E, 47 and 56° N. In Tyva, the production of dry steppes is determined on various elements of the relief – from the upper part of the mountain to the depressions located in the bottom of the slope. The value of ANP in the meadow steppes varies from west to east from 10.2 to 3.1, in the true steppes from 5.8 to 0.7 t/ha per year and depends on many factors, including air temperature, precipitation, and soil properties. The soil properties are defined by a set of conditions: the position of the ecosystem on the relief, which leads to different soil moisture. Both, the meadow and true steppes, have irregularities in the decrease of the ANP value from west to east. In some cases, in a series of meadow steppes instead of a decrease, an increase of ANP is observed, which is explaining by changes in soil conditions. The first increase from 4.8 (63° E) to 6.1 t/ha per year (73° E) occurs when Luvic Chernozem (Loamic) is replaced by Inclinigleyic Chernozem (Loamic), as a result of additional soil moistening. The second increase of ANP from 3.6 (75° E) to 6.6 t/ha per year (90° E) is observed when the soil changes from Tonguic Chernozem (Siltic) to Haplic Chernozem (Siltic, Pachic). The increases of ANP were observed in the true steppes: 1) when Skeletic Kastanozem (Siltic) changed to Calcic Chernozem (Siltic), 2) when Haplic Solonetz (Loamic) changed to Calcic Chernozem (Loamic), 3) when soil changes from Mollic Leptosol (Siltic) to Calcic Chernozem (Siltic). The value of BNP in meadow and true steppes in the soil layer of 0–30 cm generally decreases from west to east from 26.8 to 7.7 t/ha per year, varying without a visible pattern. In Tyva, due to the change in the relief, ANP of dry steppes varies from 3.7 to 1.7, BNP – from 27.0 to 8.7 t/ha per year. Consequently, the amount of aboveground production of grass ecosystems is determined not only by air temperature and precipitation, but also by the properties of soils, which vary in structure, C_org content, nutrients, and watering.