The Problem of Phased Changes in the Humus Status of Soils under Shelterbelts in Agroforestry Landscapes

Tom Sauer1*, Yury G. Chendev2, Alexander Gennadiev3, & Hamada Abdelrahman4

1 USDA-ARS National Laboratory for Agriculture and the Environment, Ames, IA2 Geologic-Geographical Faculty, Belgorod State University, Belgorod, Russia3 Department of Landscape Geochemistry and Soil Geography, Moscow

State University, Moscow, Russia 4 Department of Soil Science, Cairo University, Giza, Egypt

ABSTRACT Our studies of tree windbreaks on the Great Plains (USA) and Central Russian Upland (Russia) show the high probability of the accumulation of soil organic matter (SOM) or humus in the surface meter of soil under shelterbelts ranging in age from 21 to 70 years (Chendev et al., 2015 Eurasian Soil Sci. 48:43-53). We have reason to believe that this process has a stage character, i.e. accumulation of SOM in soils under shelterbelts does not occur indefinitely. Study of young (20-30 year-old) shelterbelts at Kamennaya Steppe in Russia in the 1920s confirmed the accumulation of SOM under the shelterbelts (Tumin, 1930 KommunaPress, Voronezh). However, research in the same areas conducted in the 1990s - 2010s showed no differences in SOM stocks in the soils of old-growth (~100 years) shelterbelts and in the adjacent background steppe areas (Kaganov, 2012 Reg. Environ. Issues 4:7-12; Prikhod’ko et al., 2013 Eurasian Soil Sci. 46:1230-40). Researchers noted that as a result of the Late Holocene forest expansion on grasslands in Europe (due to natural changes of climate), fertile Chernozem soils (in the US - Mollisols) transformed into less fertile Luvisols (in the U.S. - Alfisols) (Chendev et al., 2018 Radiocarbon 60:1185-98). Thus, the accumulation of humus in soils under shelterbelts may occur during the first decades after planting and then may be followed by a change in soil evolution including potential SOM loss. Our working hypothesis requires further research that can address an important fundamental soil-geographic problem, which consists of developing the concept of “soil formation factors — soil formation processes — soil properties”. Additional soil organic carbon (SOC) characterization including permanganate oxidizable carbon (POXC) was used to further interpret changes in SOC distribution following shelterbelt establishment. Soil cover had an insignificant effect on POXC at two sites in Russia but at a third (Yamskaya) it had a very significant effect. Depth had a significant effect on POXC at all sites (P= 0.026; <0.001 and 0.03 for Streletskaya, Yamskaya, Kamennaya, respectively). The system of “shelterbelt-soil” can act as a controlled model of interaction of soils and factors of their formation, the parameters of which are specified in a space-time dimension.

Table 1. Some physiographic characteristics of the key sites.

Figure 1. Location of key sites in U.S. Great Plains (left) and Central Russian Uplands (right).

METHODS- Pits were excavated to 1.2 m and detailed soil profile descriptions prepared under trees, crops, and native cover at each key site. Soil samples were collected from pit walls and two adjacent auger holes.- Bulk density, pH, particle size, total N and SOC (U.S. samples by dry combustion on a FisonNA 15000 Elemental Analyzer, ThermoQuest Corp., Austin, TX; Russian samples by wet combustion according to State Standard (GOST) 26213-91. were determined.- Russian soil samples were also analyzed for permanganate oxidizable organic carbon (POXC) using method of Culman et al., 2012 (Soil Sci. Soc. Am. J. 76:494-504).

HTC = hydrothermal coefficient (Selyaninov, 1928 Proceedings Agricultural Meteorology 20, 165-177).

ACKNOWLEDGEMENTS Support from the U.S. Civilian Research and Development Foundation Grant RUG1-7024-L-11. The authors are grateful for the assistance of David James (NLAE), Steve Rasmussen (Nebraska Forest Service), Jim Marten (Northeast Community College, Norfolk, NE), Dan Mager (landowner), John Hinners (South Dakota Department of Agriculture), the Beadle Soil and Water Conservation District (South Dakota), and Patrick Cowsert, Craig Stange, Martin Rosek, and Lance Howe of the Natural Resources Conservation Service (NRCS).

CONCLUSIONS - Rates of SOC accumulation beneath tree plantings were previously found to change with time, first having low rates then reaching a peak within several decades, followed by decreasing rates (Sauer et al. 2012 Plant and Soil 360:375-390).- Observed long-term loss of SOC when steppe soils are encroached by forest expansion in the Russian steppe is hypothesized to be due to this phased change in SOC accumulation combined with enhanced decomposition or movement of SOC in the soil profile (Chendev et al. 2017 Eurasian Soil Sci. 50:499-514).- Ongoing research including paleosols (buried soils) addresses mechanisms for changing rates of SOC accumulation and loss following a change in land cover from forest to grassland.

Table 2. Pools of soil organic carbon (SOC) in study soils (Mg ha-1).

Table 3. Pools of SOC in soils under the shelterbelts at the time of their planting, 0-100 cm layer (Mg ha-1).

Figure 2. Changes of SOC pools at the time of shelterbelt planting, 0-100 cm layer (Mg ha-1). Nos. 1-8 are key sites as listed in Table 1, No. 9 is data of V. Prikhod’ko et al., (2013) for 100 yr-old shelterbelt at Kamennaya Steppe.

Mg ha-1

SOC

- SOC stocks were determined by layer for tree, crop, and native grassland (Table 2).- Native grassland SOC stocks were used to estimate original SOC stocks (Table 3).- Estimated change in SOC stocks by site indicates losses and gains, likely due to time since tree planting and site characteristics (Figure 2).- Analysis of long-term forest encroachment supports hypothesis of SOC loss over long time periods (Figure 3).

POXC

- POXC is an indicator of active carbon, often correlated with SOC, particulate organic matter (POM), and microbial biomass carbon (MBC).- POXC content averaged 2.45% of SOC across all Russian sites and land uses.- POXC was significantly greater in soil beneath windbreaks at Yamskaya but no significant differences at Kamennaya or Streletskaya. - POXC was linearly correlated with SOC (average R2 = 0.63).

Figure 3. Phased changes of Chernozems into Phaeozems as a result of forest invasion of steppes during the Late Holocene.

Figure 4. Transects of POXC at Russian key sites.