Bacterial necromass as the main source of organic matter in saline soils.

Soil salinity poses a major threat to crop growth, microbial activity, and organic matter accumulation in agroecosystems in arid and semiarid regions. The limitations of carbon (C) accrual due to salinity can be partly mitigated by the application of organic fertilizers. Although microorganisms are crucial for soil organic carbon (SOC) stabilization, the relationships between living and dead microbial C pools and the community features of SOC accrual in saline soils are not known. A two-year field experiment was conducted to examine the effects of organic fertilizers on the microbial regulatory mechanisms of C sequestration in saline soil (chloride-sulfate salinity). Compared to manure addition alone, manure plus commercial humic acid increased SOC stock by 11% and decreased CO2 emissions by 10%, consequently facilitated soil C sequestration. We explain these results by greater bacterial necromass formation due to the dominance of r-strategists with faster turnover rate (growth and death), as well as larger necromass stability as supported by the increased aggregate stability under the addition of humic acids with manure. Humic acids increased the abundance of bacterial phylum Proteobacteria (copiotrophs) and decreased Acidobacteria (oligotrophs) compared with straw, indicating that r-strategists outcompeted K-strategists, leading to bacterial necromass accumulation. With larger C/N ratio (88), straw increased leucine aminopeptidase to mine N-rich substrates (i.e., from necromass and soil organic matter) and consequently reduced SOC stock by 8%. The decreased salinity and increased organic C availability under straw with manure addition also led to a 13% higher CO2 flux compared with manure application alone. Thus, humic acids added with manure benefited to SOC accumulation by raising bacterial necromass C and reducing CO2 emissions.

Soil bacterial diversity and community structure of Suaeda glauca vegetation in the Hetao Irrigation District, Inner Mongolia, China.

Exploring the bacterial community in the S. glauca rhizosphere was of great value for understanding how this species adapted to the saline-alkali environment and for the rational development and use of saline-alkali soils. In this study, high-throughput sequencing technology was used to investigate the diversity characteristics and distribution patterns of soil bacterial communities in the rhizosphere of S.glauca-dominated communities in the Hetao Irrigation Distract, Inner Mongolia, China. The relationships among bacterial characteristics, soil physicochemical properties and vegetation in four sampling sites were analyzed. The soil bacterial communities in the rhizosphere of S. glauca-dominated communities were mainly composed of 16 phyla (i.e., Proteobacteria, Actinobacteria, Bacteroidetes, Gemmatimonadetes, Chloroflexi, Acidobacteria, Firmicutes, Planctomycetes, Deinococcus-Thermus, Verrucomicrobia, Saccharibacteria, Cyanobacteria, Nitrospirae, JL-ETNP-Z39, Parcubacteria and Chlorobi), and these populations accounted for more than 99% of the total bacterial community. At the genus level, the main bacterial communities comprised Halomonas, Nitriliruptor, Euzebya and Pelagibius, which accounted for 15.70% of the total bacterial community. An alpha diversity analysis indicated that the richness and diversity of rhizosphere soil bacteria differed significantly among the sampling sites, and the bacterial richness and diversity indices of severe saline-alkali land were higher than those of light and moderate saline-alkali land. The principal component analysis (PCA) and linear discriminant analysis effect size (LEfSe) showed significant differences in the species composition of the rhizosphere soil bacterial community among different sampling sites. A correlation analysis showed that the number of bacterial species exhibited the highest correlation with the soil water content (SWC). The richness and evenness indices were significantly correlated with the SWC and SO4 2-, K+ and Mg2+ concentrations. The electrical conductivity (EC), soluble ions (Na+, CO3 2- + HCO3 -, K+, Ca2+, Mg2+, and SO4 2+), SWC and vegetation coverage (VC) were the main drivers affecting the changes in its community structure. The bacterial community in the rhizosphere of S. glauca enhanced the adaptability of S. glauca to saline-alkali environment by participating in the cycling process of nutrient elements, the decomposition of organic matter and the production of plant growth regulating substances. These results provided a theoretical reference for further study on the relationship among rhizosphere soil microorganisms and salt tolerance in halophytes.

Green manure removal with reduced nitrogen improves saline-alkali soil organic carbon storage in a wheat-green manure cropping system.

The incorporation of green manure into cropping systems is a potential strategy for sequestering soil carbon (C), especially in saline-alkali soil. Yet, there are still unknown about the substitution impacts of green manure on nitrogen (N) fertilizer in wheat-green manure multiple cropping system. Herein, a five-year field experiment was performed to determine the impact of three levels of N fertilizer inputs [i.e., N fertilizer reduced by 0 % (100N), 10 % (90 N), and 20 % (80 N)] with aboveground biomass of green manure removal (0GM) and return (100GM) on soil organic carbon (SOC) storage and its primary determinants. The results demonstrated that no significant interaction on SOC storage was detected between green manure and N fertilizer management. 80 N enhanced SOC storage in bulk soil by 7.4 and 13.2 % in 0-20 cm soil depth relative to 100 N and 90 N (p < 0.05). Regardless of N fertilizer levels, compared with 100GM, 0GM increased SOC storage in bulk soil by 14.2-34.6 % in 0-40 cm soil depth (p < 0.05). This was explained by an increase in soil macro-aggregates (>2 and 0.25-2 mm) proportion contributing to SOC physical protection. Meanwhile, the improvement of SOC storage under 0GM was due to the decrease of soil C- and N-acquisition enzyme activities, and microbial resource limitation. Alternatively, the variation partitioning analyses (VPA) results further suggested that C- and N-acquisition enzyme activities, as well as microbial resource limitation were the most important factors for SOC storage. The findings highlighted those biological factors played a dominant role in SOC accumulation compared to physical factors. The aboveground biomass of green manure removal with N fertilizer reduced by 20 % is a viable option to enhance SOC storage in a wheat-green manure multiple cropping system.

Subsurface organic amendment of a saline soil increases ecosystem multifunctionality and sunflower yield.

Salt stress poses a growing constraint to crop productivity in arid regions globally. Previous evidence indicates that organic amendment is a pivotal management practice for enhancing crop yield and soil fertility in agroecosystems. How organic amendment depth influences the interaction between soil health, ecosystem multifunctionality (EMF), and crop yield, however, still remains unclear. Thus, a 3-year field experiment was carried out to investigate the impacts of surface (0-15 cm) and subsurface (15-30 cm) applications of humic acid and manure on the soil quality index (SQI), enzyme activities, EMF, and crop yield on saline soils. Subsurface organic amendment improved the SQI (at the 0-45 cm layers) by 20-47 %, while the surface amendment improved the SQI at the 0-30 cm layer by 15-51 %. The higher soil quality under subsurface organic amendment was characterized by increases in soil organic carbon and available nutrients, and a decrease in electrical conductivity compared to surface organic amendment. The organic amendment increased microbial diversity and richness, stimulated enzyme activities, and ultimately improved soil EMF. The soil EMF increased by 122-214 % at the 0-30 cm layer under subsurface organic amendment and by only 178 % at the 0-15 cm layer under surface organic amendment. Pairwise comparisons further confirmed that electrical conductivity was negatively, and soil organic carbon positively, correlated with soil ecological functions within the 0-45 cm layer. The higher soil quality and microenvironment with better EMF under subsurface organic amendment increased sunflower yield by 16 % and 8 % as compared to inorganic fertilizer only and surface organic amendment, respectively. This relates to the considerable improvement in soil electrical conductivity, soil organic carbon, ß-glucosidase activity, and diversity and richness of microbial communities resulting from deep organic amendment. Overall, subsurface organic amendment is an effective way to enhance soil EMF and crop yield on saline soils.

Once-middle amount of straw interlayer enhances saline soil quality and sunflower yield in semi-arid regions of China: Evidence from a four-year experiment.

Straw deep returning as an interlayer is a novel practice to enhance soil carbon and nutrients. However, the impact of applying various amounts of straw as an interlayer on soil quality still remain unclear in the saline soil. Therefore, a field experiment was carried out over four years (2015-2018) in Hetao Irrigation District, China. The aim was to evaluate the impact of four straw interlayer rates (i.e., 0, 6, 12, and 18 Mg ha-1) applied at 40 cm depth on soil quality index (SQI) and its relationship to sunflower yield in saline soil. Our results showed that, in comparison to no straw interlayer (CK), straw interlayers applied at rates of 6, 12, and 18 Mg ha-1 improved SQI on average by 2.0, 2.7, and 3.0 times in four years, respectively (p < 0.05). This suggested that straw deep returning as an interlayer improved SQI, especially for middle and high amounts (12 and 18 Mg ha-1). Partial least squares path model (PLSPM) illustrated that the improvement of SQI was due to the high-moisture and low-salt environment created by straw interlayer in the early two years (2015-2016), while the higher soil nutrients released from straw decomposition in the subsequent years (2017-2018). The improvement of SQI contributed to sunflower yield, which was related to the decrease of soil salinity, the increase of soil moisture, soil organic carbon (SOC), total nitrogen (TN), and available nutrients under straw interlayers. The sunflower yield was increased by 8.7-13.4% under straw interlayers (p < 0.05), following the order of 18 = 12 > 6 >0 Mg ha-1. The greater increment of yield was detected during the initial phase of burying straw interlayers, which indicated that straw as an interlayer played a more important role than nutrient supply from straw decomposition. The findings highlighted that appropriate straw return amount (i.e., 12 Mg ha-1) as an interlayer is an economic practice to benefit soil quality and crop yield synchronously in salt-affected soils.