Polyethylene film mulching enhances the microbial carbon-use efficiency, physical and chemical protection of straw-derived carbon in an Entisol of the Loess Plateau

Biocrusts enhance soil organic carbon stability and regulate the fate of new-input carbon in semiarid desert ecosystems

Given their global prevalence, dryland (including hyperarid, arid, semiarid, and dry subhumid regions) ecosystems are critical for supporting soil organic carbon (SOC) stocks, with even small changes in such SOC pools affecting the global carbon (C) cycling. Biocrusts play an essential role in supporting C cycling in semiarid ecosystems. However, the influence of biocrusts and their successional stages on SOC and its fraction contents, as well as their role in regulating new input C into SOC fractions remain largely unknown. In this study, we collected continuous samples of bare soil (BS) and three successional stages of biocrust soils (cyanobacterial (CC), low-cover moss (LM), and high-cover moss (HM)) at 0-5 cm depth every month for one year in a semiarid desert ecosystem. We analyzed SOC changes among the samples and their fraction contents including: labile organic C (LOC) (composed of microbial biomass C (MBC), dissolved organic C (DOC), and easily oxidized organic C (EOC)) and recalcitrant organic C (ROC) fractions, soil nutrient content including: ammonium (NH4+-N), nitrate (NO3--N), and available phosphorus (AP), and soil temperature and moisture. We also conducted a 13C pulse-labelling experiment in the field to accurately quantify the effects of biocrust successional stage on exogenous C allocation to SOC fractions. Our results showed that the three successional stages of biocrust (CC-LM-HM) increased SOC and ROC contents by an average of 5.3 ± 3.6 g kg-1 and 4.0 ± 3.0 g kg-1, respectively; and the MBC, DOC, and EOC contents increased by an average of 41.7 ± 24.8 mg kg-1, 28.7 ± 12.6 mg kg-1, and 1.2 ± 0.6 g kg-1, respectively, compared to that of BS. These increases were attributed to an increase in photosynthetic pigment content, higher nutrient levels, and more suitable microclimates (e.g., higher moisture and more moderate temperature) during biocrust succession. More importantly, SOC stability was greatly improved with biocrust succession from cyanobacteria to moss, as evidenced by the reduction in soil EOC:SOC and EOC:ROC ratios by an average of 50 ± 34 % and 99 ± 67 %, respectively, while the ROC:SOC ratio increased by 33 ± 16 % with biocrust succession compared to those of BS. The biocrust SOC, DOC, and MBC 13C contents at different stages were on average 0.096 ± 0.034 mg kg-1, 0.010 ± 0.005 mg kg-1, and 0.014 ± 0.005 mg kg-1 higher than those of BS. Similarly, the allocation of new-input C among the DOC and MBC at different biocrust stages (19 ± 10 %) was significantly higher than that of BS (9 ± 6 %). New-input C into the biocrusts was fixed by microbes (43 ± 18 %) within ∼10 days and converted into other forms of C (85 ± 5 %) after 80 days. Our study provides a new perspective on how biocrusts support C cycling in semiarid desert ecosystems by mediating new C inputs into diverse fractional contents, and highlights the significance of biocrust successional stages in maintaining soil C stocks and stability in the dryland soil system.

Long-term plastic mulching decreases rhizoplane soil carbon sequestration by decreasing microbial anabolism

Ridge-furrow with plastic mulching (RFPM) is a widely used agricultural practice in rain-fed farmlands. However, the impact of microbial related metabolism on soil organic carbon (SOC) is not fully understood. Amino sugar analysis, high-throughput sequencing, and high-throughput qPCR approaches are combined to investigate this topic, based on a long-term experiment. Treatments include flat planting without mulching (FP), ridge-furrow without mulching (RF), and RFPM. RFPM significantly decreases rhizoplane SOC contents, while bulk SOC contents change insignificantly across treatments. In terms of microbial metabolic pathways, RFPM decreases indicators of the in vivo metabolic pathway, whereas those of the ex vivo pathway are increased. In terms of microbial community features, core taxa module #1 is dominated by Sphingomonadaceae. These are putative high yield (Y) strategists, according to the microbial life-history strategy framework. They are closely related to the in vivo pathway and are most predictive for SOC; their abundance is highest under FP and lowest under RFPM. Core taxa module #2 is dominated by Chitinophagaceae, putative resource acquisition (A) strategists, that are closely related to the ex vivo pathway. Their abundance in the rhizoplane is highest under RFPM and lowest under FP. The RFPM-induced decline in SOC occurs simultaneously with the abundance of A-strategists with in vivo pathway but not the Y-strategists with ex vivo pathway. Overall, the result of this study shows a trade-off. In RFPM practice, the ex vivo microbial pathway is enhanced along with the abundance of A-strategists. This is not the case for the in vivo pathway and associated abundance of Y-strategists, which are closely associated with SOC. Our findings underlined the impact of rhizoplane microbial metabolic pathways on SOC status is key to agricultural practices in drylands such as RFPM, and advanced our understanding of how microbes affect the carbon cycling in dryland farming.

In situ degradation of low-density polyethylene film in irrigation maize field: Thickness-dependent effect

Thickness of low-density polyethylene (LDPE) film might determine its mechanical strength, clean production and soil health. Yet, this issue is little understood. In situ aging effects were evaluated in LDPE films with the thickness of 0.006 mm, 0.008 mm, 0.010 mm and 0.015 mm in maize field. The data showed that maximum tensile force (TFmax), maximum tensile strength (TSmax) and elongation at break (EAB) were massively lowered with increasing thickness after aging. The greatest and lowest reduction magnitude of EAB was 27.6 % and 11.2 % in 0.006 mm and 0.015 mm films respectively. Also, the melting point (Tm) and crystallinity (Xc) under Differential Scanning Calorimeter (DSC) tended to decline with the increasing thickness. Moreover, the peak intensity of crystalline regions tended to transfer and concentrate on the amorphous regions, and such tendency became more pronounced in the thin films. Interestingly, there existed a pronounced distinct thickness-dependent effects on soil bulk density (SBD) and soil water-stable aggregate proportion. Thick plastic film mulching increased SBD but reduced the proportion of macroaggregates (mainly referred to 0.015 mm and 0.010 mm). In addition, thick film mulching slightly reduced the levels of soil organic carbon (SOC) and total nitrogen (TN), but significantly promoted the contents of soil labile C and N. Particularly, it significantly promoted above- & under-ground biomass of maize across two growing seasons (p < 0.05). To sum up, thickening LDPE film may act as a promising solution to improve LDPE film residue recycling, while benefiting for higher productivity. However, thick film mulching may cause a certain adverse impact on soil structure, and further investigations would be needed in the future.