Rhizosphere priming effects in soil aggregates with different size classes

Soil organic carbon priming co-regulated by labile carbon input level and long-term fertilization history

Labile carbon (C) input and fertilization have important consequences for soil organic matter (SOM) decomposition via the priming effect (PE), thereby impacting soil fertility and C sequestration. However, it remains largely uncertain on how the labile C input levels interact with long-term fertilization history to control PE intensity. To clarify this question, soil samples were collected from a 38-year fertilization field experiment (including five treatments: chemical nitrogen fertilizer, N; chemical fertilizer, NPK; manure, M1; 200 % manure, M2; NPK plus M2, NPKM2), with strongly altered soil physiochemical properties (i.e., soil aggregation, organic C and nutrient availability). These soil samples were incubated with three input levels of 13C-glucose (without glucose, control; low, 0.4 % SOC; high, 2.0 % SOC) to clarify the underlying mechanisms of PE. Results showed that the PE significantly increased with glucose input levels, with values increasing from negative or weak (-2.21 to 3.55 mg C g-1 SOC) at low input level to strongly positive (5.62 to 8.57 mg C g-1 SOC) at high input level across fertilization treatments. The increased PE intensity occurred along with decreased dissolved total nitrogen (DTN) contents and increased ratios of dissolved organic C to DTN, implying that the decline in N availability largely increased PE via enhanced microbial N mining from SOM. Compared to N and NPK treatments, the PE was significantly lower in the manure-amendment treatments, especially for low input level, due to more stable SOM by aggregate protection and higher N and phosphorus availability. These results suggested that manure application could alleviate SOM priming via increased soil C stability and nutrient availability. Collectively, our findings emphasize the importance of long-term fertilization-driven changes in labile C inputs, SOM stability, and nutrient availability in regulating PE and soil C dynamics. This knowledge advances our understanding of the long-term fertilization management for soil C sequestration.

Tomato growth stage modulates bacterial communities across different soil aggregate sizes and disease levels

Soil aggregates contain distinct physio-chemical properties across different size classes. These differences in micro-habitats support varied microbial communities and modulate the effect of plant on microbiome, which affect soil functions such as disease suppression. However, little is known about how the residents of different soil aggregate size classes are impacted by plants throughout their growth stages. Here, we examined how tomato plants impact soil aggregation and bacterial communities within different soil aggregate size classes. Moreover, we investigated whether aggregate size impacts the distribution of soil pathogen and their potential inhibitors. We collected samples from different tomato growth stages: before-planting, seedling, flowering, and fruiting stage. We measured bacterial density, community composition, and pathogen abundance using qPCR and 16 S rRNA gene sequencing. We found the development of tomato growth stages negatively impacted root-adhering soil aggregation, with a gradual decrease of large macro-aggregates (1-2 mm) and an increase of micro-aggregates (<0.25 mm). Additionally, changes in bacterial density and community composition varied across soil aggregate size classes. Furthermore, the pathogen exhibited a preference to micro-aggregates, while macro-aggregates hold a higher abundance of potential pathogen-inhibiting taxa and predicted antibiotic-associated genes. Our results indicate that the impacts of tomatoes on soil differ for different soil aggregate size classes throughout different plant growth stages, and plant pathogens and their potential inhibitors have different habitats within soil aggregate size classes. These findings highlight the importance of fine-scale heterogeneity of soil aggregate size classes in research on microbial ecology and agricultural sustainability, further research focuses on soil aggregates level could help identify candidate tax involved in suppressing pathogens in the virtual micro-habitats.

A holistic framework integrating plant-microbe-mineral regulation of soil bioavailable nitrogen

Soil organic nitrogen (N) is a critical resource for plants and microbes, but the processes that govern its cycle are not well-described. To promote a holistic understanding of soil N dynamics, we need an integrated model that links soil organic matter (SOM) cycling to bioavailable N in both unmanaged and managed landscapes, including agroecosystems. We present a framework that unifies recent conceptual advances in our understanding of three critical steps in bioavailable N cycling: organic N (ON) depolymerization and solubilization; bioavailable N sorption and desorption on mineral surfaces; and microbial ON turnover including assimilation, mineralization, and the recycling of microbial products. Consideration of the balance between these processes provides insight into the sources, sinks, and flux rates of bioavailable N. By accounting for interactions among the biological, physical, and chemical controls over ON and its availability to plants and microbes, our conceptual model unifies complex mechanisms of ON transformation in a concrete conceptual framework that is amenable to experimental testing and translates into ideas for new management practices. This framework will allow researchers and practitioners to use common measurements of particulate organic matter (POM) and mineral-associated organic matter (MAOM) to design strategic organic N-cycle interventions that optimize ecosystem productivity and minimize environmental N loss.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10533-021-00793-9.

Root effects on soil organic carbon: a double-edged sword

From recent developments on how roots affect soil organic carbon (SOC) an apparent paradox has emerged where roots drive SOC stabilization causing SOC accrual, but also SOC destabilization causing SOC loss. We synthesize current results and propose the new Rhizo-Engine framework consisting of two linked components: microbial turnover and the soil physicochemical matrix. The Rhizo-Engine is driven by rhizodeposition, root turnover, and plant uptake of nutrients and water, thereby accelerating SOC turnover through both stabilization and destabilization mechanisms. This Rhizo-Engine framework emphasizes the need for a more holistic approach to study root-driven SOC dynamics. This framework would provide better understanding of plant root effects on soil carbon sequestration and the sensitivity of SOC stocks to climate and land-use changes.