Understanding the determinants of soil carbon mineralization at both the aggregate and rhizosphere levels is crucial for providing precise feedback on climate change. However, the specific patterns and relative contributions of rhizosphere effects on carbon mineralization at the aggregate scale remain unclear. To address this, we conducted an incubation experiment to examine the impact of warming on soil carbon mineralization. We also assessed variations in microbial activities and soil properties across different aggregates in both rhizosphere and non-rhizosphere soils, while exploring how the rhizosphere effects modulated the response of soil respiration to warming. Our findings revealed that rhizosphere soil provides a favorable environment for microbial activities through increased nutrient input and aggregate stability, leading to significant increases in microbial biomass and enzyme activity, thus promoting soil carbon mineralization. Moreover, the spatial heterogeneity of soil aggregates contributes to the differentiation and diversity of microbial community distribution, which further enhances the diversification of carbon mineralization at the aggregate level. Notably, macroaggregates play a significant role in soil carbon flux, making a substantial contribution to carbon turnover in the ecosystem. The partial least squares (PLS) path analysis indicated that the rhizosphere positively increased the contributions of soil properties and microbial variables to the overall amount of soil cumulative mineralization, with soil nutrients playing a crucial role among these factors. These findings highlight the complex regulatory role of substrate quality in soil carbon mineralization dynamics at the aggregate scale, underscoring its far-reaching implications for accurately predicting the feedback to climate change in soil carbon mineralization. 

Soil aggregate-associated organic carbon mineralization and its driving factors in rhizosphere soil

Digestate contains many essential nutrients for crops, including nitrogen (N) and phosphorus (P), and it can alter the biogeochemical cycle of nutrients and soil functionality. This work aimed to assess the fertilizing effects of digestate on chemical and biological soil properties in a field experiment in eastern Portugal with two horticultural crops involving nine treatments: control without fertilization; mineral N fertilization with 85 kg ha−1; fertilization with digestate (DG) with increasing N rates (85, 170, 255, or 340 kg N ha−1); and fertilization with different combinations of digestate plus mineral N (DG at 85 or 170 kg N plus 60 kg mineral N ha–1 or DG at 170 kg N plus 25 kg mineral N ha–1). In addition to N, digestate supplied significant amounts of P, Ca, K, and Mg and significantly increased soil Olsen P, mineral N, and organic C. At high doses, it decreased phosphatase and β-glucosidase activities, as well as fungi and bacterial biomass, compared to the control or mineral N fertilization, and it also negatively affected soil P and C cycling capacity and microbial biomass. The organic to total N ratio and the N to P ratio in digestate are crucial properties for evaluating its agronomic management as fertilizer.

/pdf/digestate-not-only-affects-nutrient-availability-but-also-3v5sapyg.pdf

Digestate Not Only Affects Nutrient Availability but Also Soil Quality Indicators

Nitrogen availability affects the mineralization of soil organic matter differently depending on soil type and soil layer. It is unknown whether such differences result from different responses of individual carbon fraction to N supply. This study examined the priming effect (PE) on the decomposition of particulate organic matter (POM) and mineral-associated organic matter (MAOM) fractions in response to N addition. Two soils, one from a woodland and the other from cropland, were fractionated to obtain two POM fractions differing in C:N ratio and two MAOM fractions differing in mineral C protection. Two fractions of each soil were incubated for 14 days after the initial supply of 13C-glucose and Ca(NO3)2 at 0, 20 and 60 mg N kg−1. Glucose addition induced a stronger PE per unit soil organic C in the MAOM than POM, particularly in the woodland soil (6.2 vs. 0.8 mg C g−1 C). The glucose-stimulated increases in the abundances of bacteria, fungi, and four functional genes (LMCOs, ureC, npr and chiA) were also larger in the MAOM than POM. The PE was similar in the MAOM (C:N 11.6–12.4) from the two soils and was not affected by N addition. By contrast, N addition increased the PE in the POM (C:N 21.6) of the woodland soil (e.g., by 7-fold at 60 mg N kg−1), along with marked increases in the abundances of fungi, the four functional genes, and the ratio of fungi to bacteria. Compared with the woodland soil, the addition of 60 mg N kg−1 to the POM of the cropland soil (C:N 16.9) stimulated the PE to a lesser extent (by 30%). These results suggest that N input may have a greater potential in stimulating the PE in soils with a higher proportion of POM with a greater C:N ratio. 

Nitrogen addition increases the glucose-induced priming effect of the particulate but not the mineral-associated organic carbon fraction

Nitrogen (N) availability is a powerful controller of soil carbon (C) cycling in temperate forests, affecting plant productivity and microbial activities. Enhanced rhizosphere input from increased plant productivity can stimulate the decomposition of native soil organic matter (SOM), termed the “priming effect”, with an effect size that is affected by N availability. Using soils from N-fertilization field-experiments conducted in larch and mixed forest plantations, we investigated how N availability influenced the priming of soil organic C (SOC) mineralization and soil organic N (SON) mineralization by adding 13 C-labelled glucose semi-continuously (every third day) to simulate the semi-continuous delivery of rhizosphere inputs. We found that semi-continuous additions of glucose induced a repeating pattern for the priming of SOC mineralization with an initial decrease followed by an increase. This repeating pattern of SOC mineralization reversely coincided with repeating dynamics in the level of bacterial growth which first increased followed by a decrease. The labile C additions induced a gradually increasing priming of SON mineralization, which was greater than the priming of SOC mineralization (i.e. “selective microbial N-mining”). The priming of SOC mineralization and SON mineralization were both reduced by N fertilization. Increased priming of SON mineralization was related to stimulated bacterial growth and changes in the microbial C use efficiency. These responses suggested that increased microbial demand for N drove the observed N-mining responses. This microbial N-mining could not be explained by increased oxidative enzyme activities, and was instead linked to microbial growth. In the larch forest with high C-quality, the high priming of SOM mineralization were linked to bacterial growth, while in the mixed forest with low C-quality, the high levels of priming of SOM mineralization was linked to fungal growth. We also observed that labile C input initially decoupled the C and N mineralization from SOM. Within 16 days, however, a new equilibrium developed, where both C and N mineralization from SOM were similarly enhanced by labile C input. Overall, our results suggest that semi-continuous rhizosphere inputs can induce a sustained priming of SOM mineralization driven by the microbial demand for N – increasing the release of CO 2 – but that N fertilization could reduce the soil C loss, contributing to enhanced soil C sequestration. • Semi-continuous C additions primed a repeating pattern of SOC mineralization. • Glucose primed N-mining driven by microbial growth demands. • Oxidase enzyme activities did not explain the priming of SOM mineralization. • Differences in forest type SOC-quality drove distinct priming effects. • After C addition, SOC mineralization was transiently decoupled from that of SON. 

Semi-continuous C supply reveals that priming due to N-mining is driven by microbial growth demands in temperate forest plantations

Soil carbon-nitrogen (C:N) stoichiometry acts as a control over decomposition and soil organic matter formation and loss, making it a key soil property for understanding ecosystem dynamics and projected ecosystems responses to global environmental change. However, the controls of soil C:N and how they respond to increasing pressures from global change agents are not fully understood. The "foundational" controls on soil C:N, namely plant and microbial C:N, have been used to predict soil C:N, but fail to accurately simulate all ecosystems and may be insufficient for predictions under global environmental change. We present an "emerging" representation of controls of soil C:N that includes plant-microbe-mineral feedbacks that have been shown to regulate soil C:N. We argue that including representation of these emerging drivers in process-based terrestrial biogeochemistry models, which include biological N fixation, mycorrhizae, priming, root exudation of organic acids, and mineralogy (including soil texture, mineral composition, and aggregation), will improve mechanistic representation of soil C:N and associated processes. Such improvements will produce models that will better simulate a variety of ecological states and predict soil C:N when global changes modify plant-microbe-mineral interactions. Here, we align our empirical understanding of controls of soil C:N with those controls represented in models, identifying contexts where emerging drivers might be particularly important to represent (e.g., priming and root exudation in nutrient-limited conditions) and areas of future work. Additionally, we show that implementing emerging drivers of soil C:N results in different simulated outcomes at steady state and in response to elevated atmospheric CO2. Our review and preliminary simulations support the need to incorporate emerging drivers of soil C:N into process-based terrestrial biogeochemistry models, allowing for both theoretical exploration of mechanisms and potentially more accurate predictions of land biogeochemical responses to global change. 

Aligning theoretical and empirical representations of soil carbon-to-nitrogen stoichiometry with process-based terrestrial biogeochemistry models

Climate warming and shrubification will affect soil carbon (C) cycling in arctic ecosystems. Rhizosphere inputs from increased plant productivity by shrubification may stimulate the mineralization of old soil organic matter (SOM), termed the “priming effect”. However, increased soil nitrogen (N) availability due to warming-accelerated mineralization, and litter inputs associated with shrubification could modulate this response. In this study, we investigated how N-availability affects the priming of SOM mineralization in subarctic soils by adding labile organic matter (OM) including 13C-glucose with and without mineral N, or 13C-alanine, into soils with different N availabilities resulting from inorganic N and/or litter addition field-treatments. Rather than as a single pulse addition, labile OM additions were administered semi-continuously every other day to simulate rhizosphere conditions. We found that semi-continuous additions of labile OM induced a sustained priming of SOM mineralization, and this was linked to a sustained stimulation of bacterial and fungal growth over time, despite a reduced microbial growth efficiency. The priming of soil N mineralization was higher than the priming of soil C mineralization, indicating a selective microbial N-mining that was particularly pronounced in more N-poor soils. However, microbial N-mining showed a declining trend over time, suggesting a shift from the most N-rich compounds to less N-rich compounds, presumably as reservoirs were exhausted. The priming effect controlled by N-mining was associated with a stimulation of bacterial and fungal growth depending on the form of labile OM: Alanine induced higher priming of soil C mineralization by stimulating bacterial growth, while glucose induced lower priming of soil C mineralization by stimulating fungal growth. These results indicate that bacteria and fungi can both drive the priming of SOM mineralization in subarctic soils. Based on microbial biomass and growth rates, it could be estimated that over 90% of the observed priming of both soil C and N mineralization were due to changes in turnover of “old” SOM rather than of the microbial biomass pool (i.e. “real” rather than “apparent” priming). Overall, our findings suggest that increased rhizosphere inputs could increase soil N availability by enhanced microbial N-mining, generating a positive feedback to plant productivity in the subarctic. In contrast, increased soil N availability could reduce soil C release through alleviated microbial demand for N. 

Testing the environmental controls of microbial nitrogen-mining induced by semi-continuous labile carbon additions in the subarctic

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Testing the environmental controls of microbial nitrogen-mining induced by semi-continuous labile carbon additions in the subarctic