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• Compared impacts of 5 tillage systems on Calcic Cambisol erosion in Loess Plateau. • Measured soil physicochemistry, microbial diversity, and soybean yield over 9 y. • Ascomycota and Basidiomycota abundances correlated with soybean yield. • Ascomyta and Gemmatimonadetes correlated with available K and N, respectively. • BP neural network showed that ridge-furrow plastic mulching optimal for this region. Soil erosion has caused substantial nutrient losses in the hilly-gully area of the Loess Plateau in China. Consequently, traditional tillage provides little ecological benefit there, and conservation tillage is required instead. To identify a suitable conservation tillage mode for this region, we set up no-tillage (NT), straw mulching (SM), plastic mulching (PM), and ridge-furrow plastic mulching (RPM) systems, used conventional tillage (CT) as a control, and monitored all systems for nine years. We measured and compared relative soil physicochemical properties, fungal and bacterial community diversity and composition, and microbial responses to the various environmental factors. Based on the measured soil quality indicators, we used a back propagation (BP) artificial neural network to perform a comprehensive evaluation of the various treatments to identify the conservation tillage mode that provided the greatest ecological benefits. Long-term RPM was the most highly efficacious at preventing water erosion. Relative to CT, conservation tillage methods significantly increased soil available N and available K levels and, by extension, the soybean grain yields. Neither fungal nor bacterial alpha diversity indices were significantly different among the tillage treatments, but the relative abundances of the dominant bacterial and fungal phyla significantly differed among treatments, and these relative abundances were significantly correlated with both yield, soil available K, and available N contents. Based on the results of our other analyses, bulk density, water and soil conservation capacity, available N, available K, and yield were selected as the appropriate BP neural network model inputs. We then used the output, the comprehensive evaluation value of each cultivation mode, to generate comprehensive simulations. The regression coefficient of the expected and the actual output value reached 0.99. The model disclosed that all types of conservation tillage (except NT) were superior to traditional tillage and that RPM conferred the greatest comprehensive benefits. Based on BP neural network analysis, we established that ridge-furrow plastic mulching is the optimal conservation measure for the study region. The present study serves as a reference and provides guidance for selecting the ideal methods of cultivation for hillsides in the Loess Plateau of China. Our results are relevant for the mitigation of soil erosion in other similar agricultural regions worldwide. [ABSTRACT FROM AUTHOR]

Soil microbial nutrients and resource availability improve with mulching in agroecosystems, resulting in changes of soil microbial nutrient utilization strategies. However, the responses of soil microorganisms to nutrient utilization processes to regulate community assembly and thus maintain resource availability have not been clarified. Here, we performed a long-term field experiment with mulching measures, including no-mulching (NM), straw mulching (SM), plastic mulching (PM), and ridge mulching (RM), conducted on the Loess Plateau of China, to explore the community assembly process in response to microbial nutrient utilization. Microbial nutrient utilization strategies were analysed by measuring soil nutrients, microbial biomass, and extracellular enzyme activities. The results showed that PM and RM acquired C and N nutrients by secreting C- and N-extracellular enzymes in response to C:N- and C:P imbalances in soil chemistry, thereby reducing the nutrient limitation on microbial metabolism and improving microbial C utilization efficiency (CUE). Soil extracellular enzymatic stoichiometry increased and N utilization efficiency (NUE) decreased under mulching and drove an increase in beta-diversity and dominant species abundance in fungal communities. Mulching measures promoted the modular distribution of sensitive microorganisms and aggregation of fungal networks under PM and RM by reducing the stoichiometric imbalance and extracellular enzymatic N:P. At the same time, the microbial community assembly gradually changed from a stochastic to a deterministic process in response to the alleviation of soil C:P imbalance and changes in microbial nutrient utilization, in which the fungal community was dominant. In conclusion, soil microorganisms adaptively responded to changes in soil microbial nutrient utilization through the enhancement of aggregation of fungal co-occurrence networks and deterministic assembly processes. This study provides a better understanding of the impact of mulching on microbial nutrient utilization strategies from a community assembly perspective. • Microbial community composition change under mulching in loess soil was examined. • Absent mulching, community assembly was dominated by stochasticity. • Under mulching, change from stochastic to deterministic assembly was promoted. • Nutrient imbalance drove adaptive regulation of phylogenetic turnover. • Under nutrient imbalance, fungal community assembly was driven by N limitation. [ABSTRACT FROM AUTHOR]

Abstract The effects of vegetation restoration on plant characteristics and soil physicochemical properties have been widely documented; however, knowledge of the variation in soil ecoenzymatic activity and stoichiometry remains limited, particularly with respect to their relationship with plant and soil variables in restored ecosystems. Here, the vegetation and soil from one farmland and two restored land-use types (Robinia pseudoacacia plantations: RP and abandoned lands: AL) of four age classes (10, 18, 28, and 43-years) were investigated and sampled on the Loess Plateau, China. The activity of C, N, and P-acquiring enzymes (β-1,4-glucosidase, BG; β-1,4-N-acetylglucosaminidase, NAG; leucine aminopeptidase, LAP; and alkaline phosphatase, AP) and other major influencing factors (soil physical properties, soil nutrient contents, and microbial biomass) were determined. It was found that both restoration time and vegetation types significantly affect plant characteristics, soil physical properties, soil nutrient content, and microbial biomass. Soil BG, NAG + LAP, and AP enzyme activities were higher in RP and AL than in farmland, and increased with restoration time in AL. For RP, the activity of these enzymes increased from year 10 to year 28, then decreased from year 28 to year 43. Soil ecoenzymatic C:P and N:P activity ratios were ordered: farmland > AL > RP, and decreased with restoration time in AL and RP; thus, P limitation was stronger in RP than in AL and increased with restoration time in both AL and RP. Soil ecoenzymatic C:N:P acquisition ratios of farmland, AL, and RP deviated from the 1:1:1 ratio and depended on the availability of environmental nutrients and demand for microbial nutrients. The vegetation characteristics and soil physical properties were closely related to the nutrient acquisitions of microbes and, ultimately, contributed towards shaping soil ecoenzymatic activity and stoichiometry (particularly vegetation coverage, belowground biomass, soil water content, soil bulk density, and pH). Furthermore, variation in soil ecoenzymatic activity and stoichiometry was better accounted by dissolved nutrients in the soil (particularly C and N) and microbial biomass (particularly N and P) than by plant characteristics and soil properties. Overall, this study demonstrates that the C:N:P stoichiometry of soil microbes and ecoenzymatic activity is nutrient dependent, rather homeostatic, with the potential to influence nutrient cycling on the Loess Plateau. Graphical abstract Image 1 Highlights • Soil ecoenzymatic C:N:P acquisition ratios deviated from the 1:1:1. • Soil ecoenzymatic activity ratio was not in homeostasis but resource dependent. • P limitation of microbial metabolisms increased with restoration time. • Soil ecoenzymatic stoichiometry were influenced by both plant and soil properties. • Soil dissolved nutrients and microbial biomass have stronger effect on soil ecoenzymatic stoichiometry than plant and soil properties. [ABSTRACT FROM AUTHOR]

Abstract Afforestation greatly alters soil nitrogen (N) dynamics and soil microbial community, but the response of soil N dynamics to microbial activity, particularly to that of N-fixing microbes, remains unclear. In the present study, soil samples were collected from areas with a chronosequence of 17, 27 and 42 years of Robinia pseudoacacia L (RP17a, RP27a, and RP42a) and a sloped farmland (FL) as a control. The nifH gene was sequenced with the Illumina MiSeq platform to analyze soil microbial diversity and composition, and soil N fractions (TN, AN, NN, MBN, DON) were estimated. The soil N stocks, N fractions, and diversity of N-fixing microorganisms increased with the chronosequence of R. pseudoacacia. Beta diversity, analyzed using nonmetric multidimensional scaling, revealed that afforestation effectively improved soil microbial communities. The profiles of the N-fixing microorganism communities at sites 17a, 27a and 42a were clearly separated from the farmland, and soil N-fixing microbial community compositions generally changed in the farmland from Actinobacteria-dominated to Proteobacteria- dominated with stand ages. Redundancy analysis revealed that soil N fractions were positively correlated with the main species, particularly for AN, which had a 34.7% explanatory space in axis one and was significantly correlated with the Proteobacteria at the phylum level. Results implied the evolutionary tendency of the dominant microbial groups, and demonstrated the variation of the N fractions and some other physical-chemistry properties along the chronosequence of R. pseudoacacia, and that AN was the most sensitive N fraction to the N-fixing microorganism composition. Thus, our results provide evidence that the variation of soil compositions of N-fixing microbial communities is linked to the level of N fractions, especially the content of AN. Highlights • Soil diazotrophs abundance, diversity, and compositions changed along with the chronosequence. • With the increase in ages, the difference in species composition decreased. • Soil N fractions were significantly correlated with the major microbial species. • AN was the most sensitive N fraction to N-fixing microorganisms. [ABSTRACT FROM AUTHOR]

Temporal variation in soil enzyme activities has important significance for soil quality after afforestation. This study investigated the temporal variation in soil enzyme activities and their response to changes in soil properties following afforestation. Soil samples were collected during different periods of vegetation growth at sites in the Loess Plateau with different land use types: 40 year-old Robinia pseudoacacia L. (RP40a), Caragana Korshinskii Kom (CK40a), and abandoned land (AL40a), as well as millet ( Setaria italica ) farmland (FL). Activities of four enzymes (catalase, saccharase, urease and alkaline phosphatase) that are involved in C, N, P cycling and soil water content (SWC), dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and available phosphorus (AP) were measured. The results revealed that temporal variation and land-use have significant effects on the patterns of SWC, DOC, and DON, although not on that of AP. In addition, the activities of soil enzymes were highest in June, indicating that there are differences in the temporal variation of soil enzyme activities within a given land-use type, especially for alkaline phosphatase. The consistent ranking of soil enzyme activities was also significantly increased by afforestation. Furthermore, nonmetric multidimensional scaling analysis showed that the influence degree of afforestation on activities of the four enzymes was higher than that of temporal variation. Significant correlations between soil enzyme activities and soil properties indicated that soil enzyme activities are closely related to soil nutrients dynamics, particularly with respect to DON. The present study accordingly demonstrates that soil enzyme activities respond to farmland-to-forest conversion and hence have the potential to affect soil qualities in the Loess Plateau. [ABSTRACT FROM AUTHOR]

The afforestation of abandoned land could offer opportunities to sequester soil organic carbon (SOC), promote nutriment elements cycling, improve plant diversity in the plantation understory and provide ecosystem services. The objectives of this study were to identify plant diversity in the plantation understory, quantify the changes in SOC and total nitrogen (TN) storage in deep soil, assess the SOC, TN, and total phosphorus (TP) stoichiometries, and investigate their relationships in the Loess Plateau Region (LPR) undergoing long-term afforestation. Soil samples were collected at a soil depth of 0–200 cm under 30-yr old Robinia pseudoacacia L. and adjacent abandoned sites, and SOC, TN and TP were determined in different soil depth. Additionally, plant composition and diversity in the plantation understory were evaluated. The results showed that land subjected to long-term afforestation had greater plant coverage, plant density, richness index ( R ) and Shannon–Wiener diversity ( H ) compared to abandoned land communities ( P < 0.05). SOC, TN and TP contents in afforested sites were significantly increased in surface soil (0–30 cm) as well as in the underlying soil (100 cm) compared to the corresponding abandoned land sites ( P < 0.05) in most cases. Meanwhile, SOC, TN, and TP stoichiometry in afforested areas were higher than those of abandoned lands and significantly related to understory vegetation diversity ( P < 0.05). In addition, lands subjected to long-term afforestation effectively increased SOC and TN storages compared to abandoned land at soil depths of 0–30 cm and 100–200 cm and were also significantly related to understory vegetation diversity ( P < 0.05). These findings demonstrating that afforestation not only affects SOC and TN stocks in surface soil, but also strongly influences that in deep soil. And it is also indicating that long-term afforestation could greatly affect soil R CN , R CP , and R NP ratios. [ABSTRACT FROM AUTHOR]

• Afforestation varied the soil physical-chemical properties and nosZ -type microbial community structure. • Shannon and Simpson indices were strongly correlated with soil physicochemical properties. • There are some differences of dominant species in different land use types. • AN was the key environment factor to influence the composion of microorganisms. Microbial denitrification is an important part of the soil nitrogen cycling process and is the main way that naturally-fixed nitrogen returns to the atmosphere. Research to date has mainly focused on the effects of temperature, pH and carbon sources on denitrification, with the effects of different vegetation types thought to be relatively small. This study investigated the soil denitrification process in the Loess hilly region of China. Four typical land-use types were selected: farmland (FL), abandoned land (AL), Caragana korshinskii (CK), and Robinia pseudoacacia (RP). The functional gene nosZ , which regulates nitrous oxide reductase (N 2 OR), was selected as the target primer. High-throughput sequencing was carried out and the composition of soil properties was determined. The physicochemical properties of soil, including nitrate nitrogen and organic carbon levels, were studied to explore soil microbial compositions and species diversities and their main influences. Compared with FL, the content of ammonium nitrogen (AN) in AL, CK, and RP soils was 28.5%, 29.7% and 28.8% higher, respectively, while nitrate nitrogen was 1.39-, 1.96-, and 1.55-fold higher, respectively. The Shannon index showed that the microbial diversity of the nosZ gene in RP was greater than that in FL. Nonmetric multidimensional scaling analysis showed that the FL distribution was much different from those of AL, CK, and RP; meanwhile, the AL, CK, and RP distributions tended to overlap, which indicates that there were only small differences in species compositions. Redundancy analysis showed that AN is a key environmental factor affecting the nosZ type microorganisms composition in different taxonomic levels, and the contribution rates can reach 74.3% (p = 0.006), 73.2% (p = 0.004), and 62.6% (p = 0.008), respectively (from phylum to order). This study revealed the α- and β -diversities of nosZ type microorganisms and the key environmental influences on the composition of microorganisms with the land-use change. It provides a reference for further study on the heterogeneity of nitrogen output processes in different land use types. [ABSTRACT FROM AUTHOR]