Alteration of desert soil microbial community structure in response to agricultural reclamation and abandonment
Introduction
Deserts cover about one-quarter of the global land surface and are characterized by harsh climates and barren soil conditions. Reclaiming native desert ecosystems into cropland is the prevailing strategy to utilize these arid regions and combat desertification. However, in the past several decades, large areas of native deserts have been converted into cropland to feed the increasing global population (Köberl et al., 2011, Ding et al., 2013, Wei et al., 2018). For instance, the area of irrigated farmland in the Shiyang River watershed of northwestern China increased by 30% in the past 20 years, which is problematic owing to serious water shortages, soil salinization and competing demands for groundwater resources (Wei et al., 2018). In fact, high water demands are the central issue for agriculture in arid regions, which leads to significant depth declines in groundwater tables, and causes a series of eco-environmental issues (Dong et al., 2010, Wei et al., 2018). As a result, progressively more reclaimed cropland has been abandoned. Therefore, two typical contrasting types of land use (agricultural reclamation and abandonment) are ubiquitous in arid desert regions, and limited data are available to help understand the implications of their occurrences.
Soil microorganisms play key roles in maintaining plant productivity, soil fertility and health, and soil ecosystem functions by regulating many ecological processes (Biswas and Kole, 2017, Schloter et al., 2018). Microbial community diversity and structure are significantly influenced by abiotic and biotic soil environmental factors (Ramirez et al., 2010, Li et al., 2015). Thus, changes in soil physicochemical properties, such as soil moisture, pH, salinity, and nutrient availability caused by land-use changes and management, can significantly affect soil microbial community structure (Drenovsky et al., 2010, Köberl et al., 2011, Li et al., 2015, Lüneberg et al., 2018, Wang et al., 2019). However, there is no consensus on how soil microbial communities respond to farming activities (irrigation, cultivation and fertilization), with reported positive (Li et al., 2018, Wang et al., 2019), negative (Ding et al., 2013, Zhou et al., 2016) and non-significant effects (Li et al., 2015, Lazcano et al., 2013) depending on microbial taxa (Zhou et al., 2016, Wang et al., 2019) and soil type (Ramirez et al., 2010). Most previous studies have focused on typical agroecosystems (Jangid et al., 2008, Ramirez et al., 2010, Hartmann et al., 2015, Eo and Park, 2016, Hu et al., 2017), with only a few investigations on reclaimed desert agroecosystems (Köberl et al., 2011, Ding et al., 2013, Li et al., 2015). Specifically, changes to soil microbial community structures after desert farming abandonment remains poorly understood.
Soil microbial biomass, diversity and structure have been taken as indicators of soil quality that are closely related to agricultural yield (Kaschuk et al., 2010, Schloter et al., 2003, Schloter et al., 2018). Previous studies have found that farming reclamation generally enhanced soil microbial diversity in drylands due to fertilization, irrigation and cultivation (Wang et al., 2019, Köberl et al., 2011), while continuous fertilization and tillage practices tended to have opposing effects (Wang et al., 2010, Zhou et al., 2016, Wang et al., 2019). Compared to native desert soils, cultivated soils generally have higher nutrient levels owing to fertilization, crop exudates/residues and greater water contents on account of irrigation inputs, providing conditions that support more microbial growth and activity (Ding et al., 2013, Li et al., 2015). Provided that desert soil microbial communities perform important functions for erosion prevention and in ecosystem food webs (Belnap et al., 2016), it is essential to understand how reclamation alters native desert communities and if/how they rebound the following abandonment. Hence, our results will be meaningful to promote ethical land management by considering ecological impacts of both reclamation and abandonment if/when irrigation supplies cannot support the reclaimed land operations.
To access the impacts of farming reclamation and abandonment on desert soil microbial community structure, we used qPCR and Illumina MiSeq sequencing approaches to evaluate the changes in the abundances, diversities and compositions of bacterial, archaeal and fungal communities in a 10-year cultivated soil and along a 10-year farming abandonment chronosequence compared with an adjacent native desert in the southeastern edge of the Badain Jaran Desert, China. Here, we addressed the following hypotheses: 1) farming reclamation will increase soil bacterial, archaeal and fungal biomass and diversity, but this trend will reverse after farming abandonment, resulting in significantly different microbial community structures during the two contrasting processes, and 2) soil moisture and nutrient availabilities will be the key drivers in structuring desert soil microbial communities.
Section snippets
Study site and soil sampling
The study was conducted at the Minqin Desert Ecosystem National Field Observation & Research Station (38°34′28″N, 102°59′05″E) with an elevation of 1370 m a.s.l., located in the southeastern fringe of the Badain Jaran Desert, China. This area is at the lower reaches of the Shiyang River and characterized as a typical transitional belt between oasis and desert. The climate conditions include a mean annual temperature of 7.6 °C with a hot summer and a cold winter. The mean annual precipitation is
Characteristics of soil properties
Farming reclamation and abandonment significantly affected soil SM, TOC, TN, AP, C/P and N/P. Significant effects of farming reclamation on soil EC and C/N, and farming abandonment on pH, TP and DOC were also observed (Table S1). Increased SM, TOC, TN, AP, C/P and N/P were found in FS soil compared with the DS soil. In general, soil SM, TOC, TN, DOC and N/P decreased with the progression of farming abandonment though there were no significant differences among abandoned farming sites (AF2, AF5
Impacts of farming reclamation on soil chemical and microbial properties
Prior studies have found that N fertilization decreased soil pH in comparison with unmanaged desert (Li et al., 2015, Wang et al., 2019) and other non-fertilized biomes (Ramirez et al., 2010). However, we did not find significant change in soil pH after farmland reclamation, which suggests that short-term fertilization had no effects on desert soil pH in the arid region. As we expected, a significant improvement of soil fertility characterized by C and N enrichments was observed after farming
Conclusions
In summary, the land-use transformation of native desert for agriculture caused profound changes in soil physicochemical characteristics and microbial community structure in response to both farming reclamation and farming abandonment. These findings imply that desert soil microbial community structures were reshaped by cropping, irrigation, fertilization and tillage for agriculture. Some changes in microbial community structure were irreversible, as communities did not return to unmanaged
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the National Key Research and Development Program of China (2017YFC0504301), the Natural Science Foundation Committee of China (31971749), Qinghai Innovation Platform Construction Project (2017-ZJ-Y20), and the Key R&D and Transformation Projects of Qinghai Province (2019-SF-152).
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