ИСТИНА

The so called “Birch effect” (BE) is well known in soil ecology. It results in intensive rise of CO2 and nitrogen oxides release into atmosphere due to soil moistening after severe drought events, and is attributed to enchanced microbial SOM mineralization. Nevertheless the phenomenon was well known much earlier (Lebedjantzev, 1924), it was firstly experimentally explained by H.F.Birch (1954, 1958), and later named after him. The increase of initial rate of emission of soil СО2 during the natural “drying-rewetting” cycles could reach 40-fold (!) and lasts from several hours to several weeks (Birch, 1958, Unger et al., 2010, our data), although arid soils more often demonstrate 5-6 fold increase within 1-2 weeks. Although it is evident that BE is mostly due to response of soil microbiota to improved food supply, it’s physico-chemical mechanism still remains not quite clear. Earlier research focus was to reveal of the BE mechanism in order to increase the productivity of agricultural crops in arid regions, but now the focus is on the study of its role due to the growing impact of human activities to Climate Change and greenhouse emissions from natural and agro-ecosystems. It is self-important now to investigate the macro-ecological patterns of BE at regional, biome and biosphere scales, i.e., to highlight it’s large-scale controls, and to estimate the impact of “drying-rewetting” weather cycles, associated with contemporary climate change and growing land use, to annual carbon budget. Rise of CO2 emission is the most important and easily determined external symptom of BE. In terms of carbon balance and greenhouse effect in the biosphere, the “old” soil carbon, which emitted into the atmosphere sporadically by microbial respiration (i.e. due to BE), is more important than carbon, respired by roots, because the latter is directly compensated by photosynthesis. BE is known to be mainly attributed to the so-called "water-controlled" ecosystems, i.e., the ones with carbon balance mostly determined by amount and timing of water input, not by temperature (Zhang et al., 2008). In these ecosystems seasonal droughts are normal. Nevertheless the evidence is accumulating, that BE is more widespread than it was previously thought. In particular, it could be also widely meet in "temperature-controlled" communities, for instance, in boreal forests (Borken et al., 1999, Werner et al., 2002), taiga (Larionova et al., 2010), and tundra ecosystems (Laporte et al., 2002, Illeris et al., 2003, Karelin et al., 2016, in press). However, the data on BE from low-temperature regions are very scarce. Besides, there are no studies summarizing the available data at the biome scale, including croplands. The goal of this new Project is to perform a statistical analysis of macro-ecological patterns, factors and emission potential of BE, based on CO2 emission source data being collected using the same technique and methods, in different natural and human affected ecosystems. Here we represent first results related to the study of self-restoration of idle arable lands in main geographic zones of European Russia. The most of existing Russian fallows were formed within the period of 1990 – 2007 after the collapse of the former Soviet Union. This supposed to be one of the greatest such an “experiments” on arable land abandonment in the human history. The total area of fallows in Russian now is estimated as 65 mln ha (Luyri et al., 2010), which is equal to the area of France. Therefore, the study of the patterns of soil emissions from this huge array of fallows during self-restoration, seems to be very important itself. Research area and methods. This chronosequence longitudinal (58 – 48oN) study in European Russia included measurements of soil respiration and changes of “Birch effect” during self-restoration of south taiga on Pogzols (tillage, 7, 23, 55, 100, 170 yr after abandonment; Valdai region); temperate mixed forest zone (tillage, 2, 28, yr; Smolensk region); temperate broadleaf forest on Haplic Luvisols (tillage, 3, 7, 20, 60, 110 yr; Orel region); forest-steppe on Chernozems (tillage, 2, 8, 38, 66 yr; Kursk region), and dry steppe on Calcisol–Solonetz complexes (tillage, 7, 12, 17 and 42 yr; Astrahan’ region). Sites are located in Fig. 1. Measurements of the BE in the ecosystems were done in 2012 - 2015. CO2 emissions were measured monthly from April till September at every succession stage, using closed (opaque) chamber technique. Respiration rates were estimated before and after 200 ml of distilled water addition per base (equivalent of 21 mm rain event). For a given chamber area (95 cm2) field water capacity of the soil in the top 6-7 cm is normally reached. After the infiltration of water into the soil we waited 30 min, and repeated the measurements. Temperature and volumetric moisture content of the upper 10 cm of soil were recorded, also we took soil samples for chemical analysis. Maximum rates of observed CO2 soil emission fluxes due to BE vs. initial respiration were than compared between stages of successions and different zonal ecosystems. RESULTS. The succession stages in dry steppe demonstrate the lowest seasonal rate of basic soil respiration at average (Fig. 2): it was 3.5 times lower, if compared to the fallows chronosequence in Podzols, 4.6 times lower, if compared to Chernozems, and the maximum difference was found with the Haplic Livisols in the zone of broadleaf forests – 7.8 times lower. The BE for soil respiration of the zonal fallow chronosequence in study was shown to increase from 1.1 ± 0.6 (N = 70) for broadleaf Haplic Vertisols (no effect), to 7.1 ± 1.5 in forest steppe ecosystems, and to 15.2 ± 3.5 in dry steppe, which is due to progressive aridity and deterioration of hydrothermal conditions. In south taiga zone it was weakly expressed and changed within the limits of 1.0 - 2.5. We assume that the lack of BE in greyzems of broadleaf forest zone, as well as the lack of relation between BE index and the age of fallows, is due to the most favorable hydrothermal conditions in the zone of deciduous broadleaf forests, in particular, to the absence of periods of summer droughts longer than 6 days. It was also not found in this chronosequence any significant relationship between the BE index and the Selyaninov Index (ratio of precipitation in mm per period (multiplied by 10) to sum of positive degree-days with mean daily temperatures above 10°C during the same period). This is because the Selyaninov Index characterises the average hydrothermal conditions during the period, but it doesn’t reflect their variation. However, if analyze the relationship between the average gain in CO2 soil emission due to BE in the different geographical zones and the number of days without precipitation during the same observation period (May – August), even on such a small array the correlation is significant for the number of drought periods per season, lasting 6 or more days, and it remains significant for the number of periods from 7 till 12 days of drought long (Fig. 2). In all studied fallow successions the BE was mostly expressed in the period with the lowest monthly Selyaninov Index (July). But only in Chernozems and dry steppe soil complexes the effect was significantly greater at the late stages of successions (Fig. 3). This cannot be attributed to specific reserves of organic matter in the particular type of soil, as it brings together the series of observations with the highest and the lowest carbon reserves. Possibly, under conditions of summer aridity, which is characteristic for dry steppe and chernozems, this is due to a greater intensity of the so-called "Gadgil effect" (Gadgil and Gadgil, 1971), associated with rise in competition between microbes and roots, when the root mass of grasses and dwarf shrubs increases at the late stages of fallow successions.