Grassland degraded patchiness reduces microbial necromass content but increases contribution to soil organic carbon accumulation.

Plant and microbially derived carbon (C) are the two major sources of soil organic carbon (SOC), and their ratio impacts SOC composition, accumulation, stability, and turnover. The contributions of and the key factors defining the plant and microbial C in SOC with grassland patches are not well known. Here, we aim to address this issue by analyzing lignin phenols, amino sugars, glomalin-related soil proteins (GRSP), enzyme activities, particulate organic carbon (POC), and mineral-associated organic carbon (MAOC). Shrubby patches showed increased SOC and POC due to higher plant inputs, thereby stimulating plant-derived C (e.g., lignin phenol) accumulation. While degraded and exposed patches exhibited higher microbially derived C due to reduced plant input. After grassland degradation, POC content decreased faster than MAOC, and plant biomarkers (lignin phenols) declined faster than microbial biomarkers (amino sugars). As grassland degradation intensified, microbial necromass C and GRSP (gelling agents) increased their contribution to SOC formation. Grassland degradation stimulated the stabilization of microbially derived C in the form of MAOC. Further analyses revealed that microorganisms have a C and P co-limitation, stimulating the recycling of necromass, resulting in the proportion of microbial necromass C in the SOC remaining essentially stable with grassland degradation. Overall, with the grassland degradation, the relative proportion of the plant component decreases while than of the microbial component increases and existed in the form of MAOC. This is attributed to the physical protection of SOC by GRSP cementation. Therefore, different sources of SOC should be considered in evaluating SOC responses to grassland degradation, which has important implications for predicting dynamics in SOC under climate change and anthropogenic factors.

Ao D ，Wang B ，Wang Y ，Chen Y ，Anum R ，Feng C ，Ji M ，Liang C ，An S ... -  《-》

Phosphorus addition decreases plant lignin but increases microbial necromass contribution to soil organic carbon in a subalpine forest.

Increasing phosphorus (P) inputs induced by anthropogenic activities have increased P availability in soils considerably, with dramatic effects on carbon (C) cycling and storage. However, the underlying mechanisms via which P drives plant and microbial regulation of soil organic C (SOC) formation and stabilization remain unclear, hampering the accurate projection of soil C sequestration under future global change scenarios. Taking the advantage of an 8-year field experiment with increasing P addition levels in a subalpine forest on the eastern Tibetan Plateau, we explored plant C inputs, soil microbial communities, plant and microbial biomarkers, as well as SOC physical and chemical fractions. We found that continuous P addition reduced fine root biomass, but did not affect total SOC content. P addition decreased plant lignin contribution to SOC, primarily from declined vanillyl-type phenols, which was coincided with a reduction in methoxyl/N-alkyl C by 2.1%-5.5%. Despite a decline in lignin decomposition due to suppressed oxidase activity by P addition, the content of lignin-derived compounds decreased because of low C input from fine roots. In contrast, P addition increased microbial (mainly fungal) necromass and its contribution to SOC due to the slower necromass decomposition under reduced N-acquisition enzyme activity. The larger microbial necromass contribution to SOC corresponded with a 9.1%-12.4% increase in carbonyl C abundance. Moreover, P addition had no influence on the slow-cycing mineral-associated organic C pool, and SOC chemical stability indicated by aliphaticity and recalcitrance indices. Overall, P addition in the subalpine forest over 8 years influenced SOC composition through divergent alterations of plant- and microbial-derived C contributions, but did not shape SOC physical and chemical stability. Such findings may aid in accurately forecasting SOC dynamics and their potential feedbacks to climate change with future scenarios of increasing soil P availability in Earth system models.

Luo R ，Kuzyakov Y ，Zhu B ，Qiang W ，Zhang Y ，Pang X ... -  《-》

Divergent contribution of particulate and mineral-associated organic matter to soil carbon in grassland.

Sequestration of soil organic carbon (SOC) is an effective means to draw atmospheric CO2. Grassland restoration is one of the fastest methods to increase soil C stocks, and particulate-associated C and mineral-associated C play critical roles in soil C stocks during restoration. Herein, we developed a conceptual mechanistic frame regarding the contributions made by mineral-associated organic matter to soil C during the restoration of temperate grasslands. Compared to 1-year grassland restoration, 30-year restoration increased mineral-associated organic C (MAOC) by 41% and particulate organic C (POC) by 47%. The SOC changed from microbial MAOC predominance to plant-derived POC predominance, as the POC was more sensitive to grassland restoration. The POC increased with plant biomass (mainly litter and root biomass), while the increase in MAOC was mainly caused by the combined effects of increasing microbial necromass and leaching of the base cations (Ca-bound C). Plant biomass accounted for 75% of the increase in POC, whereas bacterial and fungal necromass contributed to 58% of the variance in MAOC. POC and MAOC contributed to 54% and 46% of the increase in SOC, respectively. Consequently, the accumulation of the fast (POC) and slow (MAOC) pools of organic matter are important for the sequestration of SOC during grassland restoration. Overall, simultaneous tracing of POC and MAOC helps further understand the mechanisms and predict soil C dynamics combined with the input of plant C, microbial properties, and availability of soil nutrients during grassland restoration.

Liao J ，Yang X ，Dou Y ，Wang B ，Xue Z ，Sun H ，Yang Y ，An S ... -  《-》

Vegetation restoration altered the soil organic carbon composition and favoured its stability in a Robinia pseudoacacia plantation.

Soil organic carbon (SOC) stabilization is vital for the mitigation of global climate change and retention of soil carbon stocks. However, there are knowledge gaps on how SOC sources and stabilization respond to vegetation restoration. Therefore, we investigated lignin phenol and amino sugar biomarkers, SOC physical fractions and chemical structure in one farmland and four stands of a Robinia pseudoacacia plantation. We observed that the content of SOC increased with afforestation, but the different biomarkers had different contributions to SOC. Compared to farmland, the contribution of lignin phenols to SOC decreased in the plantations, whereas there was no difference among the four stand ages, likely resulting from the balance between increasing lignin derivation input and increasing lignin degradation. Conversely, vegetation restoration increased the content of microbial necromass carbon (MNC) and the contribution of MNC to SOC, mainly because microbial residue decomposition was inhibited by decreasing the activity of leucine aminopeptidase, while microbial necromass preservation was promoted by adjusting soil variables (soil water content, clay, pH and total nitrogen). In addition, vegetation restoration increased the particulate organic carbon (POC), mineral-associated organic carbon (MAOC) pools and the O-alkyl C intensify. Overall, vegetation restoration affected SOC composition by regulating lignin phenols and microbial necromass and also altered SOC stabilization by increasing the physically stable MAOC pool during late afforestation. The results of this study suggest that more attention should be given to SOC sequestration and stability during late vegetation restoration.

Su Z ，Zhong Y ，Zhu X ，Wu Y ，Shen Z ，Shangguan Z ... -  《-》

Differential carbon accumulation of microbial necromass and plant lignin by pollution of polyethylene and polylactic acid microplastics in soil.

The wide microplastics (MPs) occurrence affects soil physicochemical and biological properties, thereby influencing its carbon cycling and storage. However, the regulation effect of MPs on soil organic carbon (SOC) formation and stabilization remains unclear, hindering the accurate prediction of carbon sequestration in future global changes under continuous MP pollution. Phospholipid fatty acids, amino sugars and lignin phenols were used in this study as biomarkers for microbial community composition, microbial necromass and plant lignin components, respectively, and their responses to conventional (polyethylene; PE) and biodegradable (polylactic acid; PLA) MPs were explored. Results showed PLA MPs had positive effects on soil microbial biomass, while the positive and negative effects of PE MPs on microbial biomass varied with MP concentration. PE and PLA MPs increased microbial necromass contents and their contribution to SOC, mainly due to the increase in fungal necromass. On the contrary, PE and PLA MPs reduced lignin phenols and their contribution to SOC, mainly owing to the reduction in vanillyl-type phenols. The response of microbial necromass to PLA MPs was higher than that to PE MPs, whereas the response of lignin phenols was the opposite. MPs increased SOC level, with 83%-200% and 50%-75% of additional SOC in PE and PLA treatments, respectively, originating from microbial necromass carbon. This finding indicates that the increase in SOC pool in the presence of MPs can be attributed to soil microbial necromass carbon, and MPs increased capacity and efficacy of microbial carbon pump by increasing microbial turnover and reducing microbial N limitation. Moreover, the increase in amino sugars to lignin phenols ratio in PE treatment was higher than that in PLA treatment, and the increase in SOC content in PLA treatment was higher than that in PE treatment, indicating a high possibility of SOC storage owing to PLA MPs.

Yu H ，Liu H ，Yang K ，Xi B ，Tan W ... -  《-》