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We compared soil moisture content, pH, total organic carbon (C org), total nitrogen (TN), total phosphorus (TP) and inorganic N (NH₄ ⁺-N, NO₃ ⁻-N) concentrations, soil potential C and N mineralization rates, soil microbial biomass C (C mic), soil metabolic quotient (qCO₂), soil microbial quotient (C mic/C org) and soil enzyme (urease and invertase) activities in semiarid sandy soils under three types of land cover: grassland, Mongolian pine (Pinus sylvestris var. mongolica) plantation, and elm (Ulmus punila)-grass savanna in southeastern Keerqin, in northeast China. Soil C org, TN and TP concentrations (0-10, 10-20, 20-40 and 40-60 cm) were lower while soil C/N and C/P ratios were higher in the plantation than in grassland and savanna. The effects of land cover change on NH₄ ⁺-N and NO₃ ⁻-N concentrations, soil potential nitrification and C mineralization rates in the surface soil (0-10 cm) were dependent on sampling season; but soil potential N mineralization rates were not affected by land cover type and sampling season. The effects of land cover change on C mic and qCO₂ of surface soil were not significant; but C mic/C org were significantly affected by land cover change and sampling season. We also found that land cover change, sampling season and land cover type x sampling season interaction significantly influenced soil enzyme (urease and invertase) activities. Usually soil enzyme activities were lower in the pine plantations than in grassland and savanna. Our results suggest that land cover change markedly influenced soil chemical and biological properties in sandy soils in the semiarid region, and these effects vary with sampling season.

Heavy metal bioavailability depends on metal fractions in soil. The impacts of mild wheat residue (<2 mm) and incubation time on fractions of Cd were studied in two different spiked soils sampled from Hamadan and Lahijan, Iran with semiarid and temperate climates, respectively. Two factorial experiments were done in two soils polluted with 10 μg Cd g −1 soil separately. Organic matter (0 and 5 % wheat straw) and soil incubation time (24 and 3600 h) were factors examined in three replicates. The transformation of Cd from KNO 3 extractable form to less available fractions was higher in semiarid soils with lower clay and OM contents and higher pH and carbonate contents compared to temperate soils. In polluted semiarid soils after 24 h incubation, greater content of Cd was observed in residual (HNO 3 extractable) (45 %), carbonates associated (EDTA extractable) (34 %), organic matter associated (NaOH extractable) (11 %), and KNO 3 extractable (10 %) fractions, but in temperate soils, greater content of Cd was observed in KNO 3 extractable (61 %), HNO 3 extractable (14 %), EDTA extractable (13 %), and NaOH extractable (12 %) fractions. KNO 3 extractable form of Cd was decreased, and NaOH extractable and HNO 3 extractable forms of Cd were increased by addition of wheat residue to both soils. The initial decrease of added Cd from KNO 3 extractable form to less mobile fractions in Hamadan soil was very interesting. But this change was not observed in Lahijan soil. Since contamination factor was significantly high in temperate soils compared to semiarid soils in all treatments, the risk of Cd environmental pollution in temperate region is considerably high.

Replacement of fallow in crop–fallow systems with cover crops (CCs) may improve soil properties. We assessed whether replacing fallow in no‐till winter wheat (Triticum aestivum L.)–fallow with winter and spring CCs for 5 yr reduced wind and water erosion, increased soil organic carbon (SOC), and improved soil physical properties on a Ulysses silt loam (fine‐silty, mixed, superactive, mesic Aridic Haplustolls) in the semiarid central Great Plains. Winter triticale (×Triticosecale Wittm.), winter lentil (Lens culinaris Medik.), spring lentil, spring pea (Pisum sativum L. ssp.), and spring triticale CCs were compared with wheat–fallow and continuous wheat under no‐till management. We also studied the effect of triticale haying on soil properties. Results indicate that spring triticale and spring lentil increased soil aggregate size distribution, while spring lentil reduced the wind erodible fraction by 1.6 times, indicating that CCs reduced the soil's susceptibility to wind erosion. Cover crops also increased wet aggregate stability and reduced runoff loss of sediment, total P, and NO3–N. After 5 yr, winter and spring triticale increased SOC pool by 2.8 Mg ha–1 and spring lentil increased SOC pool by 2.4 Mg ha–1 in the 0‐ to 7.5‐cm depth compared with fallow. Triticale haying compared with no haying for 5 yr did not affect soil properties. Nine months after termination, CCs had, however, no effects on soil properties, suggesting that CC benefits are short lived in this climate. Overall, CCs, grown in each fallow phase in no‐till, can reduce soil erosion and improve soil aggregation in this semiarid climate.

BackgroundLand use/cover and management practices are widely known to influence soil organic matter (SOM) quality and quantity. The present study investigated the effect of different land use, i.e., forests viz. mixed forest cover (MFC), Prosopis juliflora (Sw.) DC-dominated forest cover (PFC), and cultivated sites viz. agriculture field (AF), vegetable field (VF), respectively, on soil parameter, microbial activity, and enzymes involved in soil nutrient cycle in a semiarid region of India.ResultsThe results showed a significant reduction (P < 0.05) in soil carbon (SC), soil nitrogen (SN) content (~ 30–80%) and consequently the soil microbial biomass carbon (SMBC) (~ 70–80%), soil basal respiration (SBR), soil substrate-induced respiration (SSIR), and soil enzyme activities (β-glucosidase, acid phosphatase, and dehydrogenase) under cultivated sites in comparison with forest sites. Pearson’s correlation showed that a positive correlation of SC with SMBC, SBR, SSIR (P < 0.01), and enzymatic activities (i.e., β-glucosidase, dehydrogenase) (P < 0.05) may imply the critical role of SC in regulating microbial and enzymatic activity. Also, a positive correlation of soil moisture with urease activity (P < 0.01) was found suggesting it as a significant abiotic factor for soil biological functions. Additionally, based on the PCA analysis, we observed the clustering of SMBC/SC ratio and qCO2 nearby AF.ConclusionOur study suggests that soil microbial parameters (SMBC, SBR, SSIR, SMBC/SC, qCO2) and enzyme activity are key indicators of soil health and fertility. Land use/cover alters the SOM content and soil microbial functions. The management strategies focusing on the conservation of natural forest and minimizing the land disturbances will be effective in preventing soil carbon flux as CO2 and maintaining the SC stock.

Land degradation by deforestation adversely impacts soil properties, and long-term restoration practices have been reported to potentially reverse these effects, particularly on soil microorganisms. However, there is limited knowledge regarding the short-term effects of restoration on the soil bacterial community in semiarid areas. This study evaluates the bacterial community in soils experiencing degradation (due to slash-and-burn deforestation) and restoration (utilizing stone cordons and revegetation), in comparison to a native soil in the Brazilian semiarid region. Three areas were selected: (a) under degradation; (b) undergoing short-term restoration; and (c) a native area, and the bacterial community was assessed using 16S rRNA sequencing on soil samples collected during both dry and rainy seasons. The dry and rainy seasons exhibited distinct bacterial patterns, and native sites differed from degraded and restoration sites. Chloroflexi and Proteobacteria phyla exhibited higher prevalence in degraded and restoration sites, respectively, while Acidobacteria and Actinobacteria were more abundant in sites undergoing restoration compared to degraded sites. Microbial connections varied across sites and seasons, with an increase in nodes observed in the native site during the dry season, more edges and positive connections in the restoration site, and a higher occurrence of negative connections in the degradation site during the rainy season. Niche occupancy analysis revealed that degradation favored specialists over generalists, whereas restoration exhibited a higher prevalence of generalists compared to native sites. Specifically, degraded sites showed a higher abundance of specialists in contrast to restoration sites. This study reveals that land degradation impacts the soil bacterial community, leading to differences between native and degraded sites. Restoring the soil over a short period alters the status of the bacterial community in degraded soil, fostering an increase in generalist microbes that contribute to enhanced soil stability.

The residual effects of adding 40 t ha^sup -1^ sewage sludge (SL) to a degraded soil cropped with barley were investigated after 9 and 36 months in a field experiment under semiarid conditions. The principal soil properties were apparently still affected by SL amendment 9 months after application but the effects disappeared after 36 months. With respect to control soil humic acids (HAs), the SL-HA was characterized by higher contents of S- and N-containing groups, smaller contents of acidic groups, a prevalent aliphaticity, extended molecular heterogeneity, and smaller degrees of aromatic polycondensation and humification. Amendment with SL caused an increase in N, H, S and aliphaticity contents and a decrease in C/N ratios and O and acidic functional group contents in soil HAs isolated 9 months after SL application. These effects tended to decrease after 36 months, most probably because the slightly humified SL-HA was mineralised over time through extended microbial oxidation, while only the most recalcitrant components such as S-containing and aromatic structures were partially accumulated by incorporation into soil HA. Microbial biomass, basal respiration, metabolic quotient and enzymatic activities increased in soil 9 months after SL application, possibly because of increased soil microbial metabolism and enhanced mineralisation processes. After 36 months these properties returned to values similar to those of the unamended soil, presumably due to the loss of energy sources.[PUBLICATION ABSTRACT]

Termites and soil calcium carbonate are major factors in the global carbon cycle: termites by their role in decomposition of organic matter and methane production, and soil calcium carbonate by its storage of atmospheric carbon dioxide. In arid and semiarid soils, these two factors potentially come together by means of biomineralization of calcium carbonate by termites. In this study, we evaluated this possibility by testing two hypotheses. Hypothesis 1 states that termites biomineralize calcium carbonate internally and use it as a cementing agent for building aboveground galleries. Hypothesis 2 states that termites transport calcium carbonate particles from subsoil horizons to aboveground termite galleries where the carbonate detritus becomes part of the gallery construction. These hypotheses were tested by using (1) field documentation that determined if carbonate-containing galleries only occurred on soils containing calcic horizons, (2) ¹³C/¹²C ratios, (3) X-ray diffraction, (4) petrographic thin sections, (5) scanning electron microscopy, and (6) X-ray mapping. Four study sites were evaluated: a C₄-grassland site with no calcic horizons in the underlying soil, a C₄-grassland site with calcic horizons, a C₃-shrubland site with no calcic horizons, and a C₃-shrubland site with calcic horizons. The results revealed that carbonate is not ubiquitously present in termite galleries. It only occurs in galleries if subsoil carbonate exists within a depth of 100 cm. ¹³C/¹²C ratios of carbonate in termite galleries typically matched ¹³C/¹²C ratios of subsoil carbonate. X-ray diffraction revealed that the carbonate mineralogy is calcite in all galleries, in all soils, and in the termites themselves. Thin sections, scanning electron microscopy, and X-ray mapping revealed that carbonate exists in the termite gut along with other soil particles and plant opal. Each test argued against the biomineralization hypothesis and for the upward-transport hypothesis. We conclude, therefore, that the gallery carbonate originated from upward transport and that this CaCO₃ plays a less active role in short-term carbon sequestration than it would have otherwise played if it had been biomineralized directly by the termites.

Cyanobacteria are a key biotic component as primary producers in biocrusts, topsoil communities that have important roles in the functioning of drylands. Yet, major knowledge gaps exist regarding the composition of biocrust cyanobacterial diversity and distribution in Mediterranean ecosystems. We describe cyanobacterial diversity in Mediterranean semiarid soil crusts along an aridity gradient by using next-generation sequencing and bioinformatics analyses, and detect clear shifts along it in cyanobacterial dominance. Statistical analyses show that temperature and precipitation were major parameters determining cyanobacterial composition, suggesting the presence of differentiated climatic niches for distinct cyanobacteria. The responses to temperature of a set of cultivated, pedigreed strains representative of the field populations lend direct support to that contention, with psychrotolerant vs thermotolerant physiology being strain dependent, and consistent with their dominance along the natural gradient. Our results suggest a possible replacement, as global warming proceeds, of cool-adapted by warm-adapted nitrogen-fixing cyanobacteria (such as Scytonema) and a switch in the dominance of Microcoleus vaginatus by thermotolerant, novel phylotypes of bundle-forming cyanobacteria. These differential sensitivities of cyanobacteria to rising temperatures and decreasing precipitation, their ubiquity, and their low generation time point to their potential as bioindicators of global change.

Afforestation of marginal agricultural land has been considered to be an effective measure to sequester atmospheric CO₂. In this study, we adopted the volume- and mass-based methods to investigate the changes in soil organic C and total N stocks in 100 cm depth following afforestation of marginal agricultural land using a chronosequence of poplar (Populus euramericana cv. “N3016”) stands in a semiarid region of Liaoning Province, Northeast China. Our results showed that soil organic C and total N concentrations in 45-60 cm layer increased gradually following afforestation of agricultural land, whereas in 60-100 cm layer, they declined initially, and then increased with stand development. Based on volume- and mass-based methods, such land-use change caused initial declines in soil organic C and total N stocks, and then increases between the stand ages of 10 and 20. Forest soils recovered to the initial soil organic C and N stocks found in agricultural land at age 15. However, the volume-based method would underestimate the absolute organic C and N stocks compared with the mass-based methods. Our results suggest that afforestation of marginal agricultural land has the potential to sequester atmospheric CO₂ in soils in semiarid regions. Stand age, soil sampling depth and the methods used to quantify organic C and N stocks should be considered for accurate assessments of changes in soil organic C and N stocks.

Cover crops are increasingly adopted to improve soil health in arid and semiarid regions, yet their effects on soil profile organic carbon (C) and nitrogen (N) and crop yield are inconsistent. We evaluated the cover crop effect on soil organic C (SOC) and N (SON) contents and water-filled pore space to a depth of 0.8 m and crop yield in a winter wheat ( Triticum aestivum L.)–sorghum ( Sorghum bicolor L. Moench)–fallow rotation under limited-irrigation conditions. Cover crop treatments were fallow (no cover crop), pea ( Pisum sativum L.), oat ( Avena sativa L.), canola ( Brassica napus L.), pea-canola mixture, oat-pea mixture, pea-oat-canola mixture, and a six-species mixture including pea, oat, canola, hairy vetch ( Vicia villosa Roth), forage radish ( Raphanus sativus L.), and barley ( Hordeum vulgare L.). Five years of cover cropping (2016–2020) did not affect SOC storage. Soil organic N at equivalent soil mass (ESM) layer 0–2500 Mg ha −1 was 8–14% greater in fallow than other treatments, except pea and oat-pea mixture. The fallow treatment also had 54–156% and 11–72% higher inorganic N content than cover crop treatments at ESM layers of 2500–5000 and 5000–7500 Mg ha −1 , respectively. Sorghum grain yield was 33–97% higher following fallow and oat as cover crop than other treatments in 2020. Although there was a variation in crop yield responses, cover cropping largely did not affect soil profile C and N contents under a limited-irrigation semiarid cropping system.