Svenja C. Stock

Background and aims: Agricultural soils in semi-arid sub-Saharan Africa are often low in soil organic carbon (SOC) and nitrogen (N), making them vulnerable to degradation and threatening food security of smallholder farmers. Conservation agriculture practices such as mulching are promoted as climate-resilient strategies to improve soil fertility, but it remains unclear whether these practices improve soil biogeochemical properties in semi-arid regions. Methods: We conducted a greenhouse experiment that simulated a cowpea ( Vigna unguiculata ) growth period, a regional key crop, under water optimum and stress conditions using sandy soil from the Omusati region in Namibia. Effects of different mulches (wheat straw, wood chips, 1:1 mixture), inoculation with a locally adapted Bradyrhizobium and water conditions on soil biogeochemical properties, including SOC, N, microbial biomass and enzyme kinetics (β-glucosidase, chitinase, leucine-aminopeptidase) were investigated. Results: We showed that mulching had limited short-term effects on SOC and N. Inoculation with rhizobia increased soil N, lowered C:N ratios and reduced N-related enzyme activities, demonstrating its potential to mitigate N limitation. Water stress decreased enzyme activities by 64%, indicating that water availability may outweigh management effects. Inoculation shifted the microbial demand from N towards C under water optimum, whereas all soils were N limited under water stress, reflected in enzymatic C:N ratios. Conclusion: Our findings highlight cowpea inoculation as an effective strategy to improve soil N, while mulching provides additional benefits for soil fertility only under favourable water conditions. Increased C and N contents show that combining inoculated legumes with mulching can enhance nutrient availability in semi-arid agroecosystems, contributing to sustainable land management.

Despite increasing climatic stress, grassland management is intensifying globally to enhance productivity. However, how increased biomass removal and associated shifts in plant composition and plant–microbe feedbacks regulate ecosystem functioning, particularly in nutrient-poor tropical grasslands, remains poorly understood. We hypothesised that increasing biomass removal structures plant communities and their rhizomicrobiome from resource-conservative towards resource-acquisitive dominated systems, and that both low and high management intensities induce unfavourable feedbacks between nutrient cycling and resource allocation, constraining soil microbial activity, plant productivity, and drought resilience. In a native managed tropical grassland, four aboveground biomass removal frequencies (1×, 2×, 3×, and 6× cutting annually) were applied in the field. Intact soil core mesocosms were exposed to drought stress in a split-plot completely randomised design (CRD), and combined with 13CO2 pulse labelling. We assessed plant biomass yield, 13CO2 assimilation and belowground C allocation, arbuscular mycorrhizal fungi (AMF) colonisation, extracellular enzyme activities, and rhizosphere microbial community structure and function. Increased cutting frequency shifted vegetation from resource-conservative, “slow” strategists to more acquisitive, “fast” strategists, accompanied by a transition from oligotrophic, fungi-dominated to copiotrophic, bacteria-dominated rhizosphere communities. Drought reduced total 13C assimilation and biomass production but enhanced allocation of assimilates to symbiotic and stress-alleviating microbes, particularly AMF. Overall, our results demonstrate that biomass removal regulates plant–soil–microbe interactions underpinning ecosystem resilience and productivity under drought, highlighting the importance of optimising management intensity in tropical grasslands under climate change.

Despite increasing climatic stress, grassland management is intensifying globally to enhance productivity. However, how increased biomass removal and associated shifts in plant composition and plant–microbe feedbacks regulate ecosystem functioning, particularly in nutrient-poor tropical grasslands, remains poorly understood. We hypothesised that increasing biomass removal structures plant communities and their rhizomicrobiome from resource-conservative towards resource-acquisitive dominated systems, and that both low and high management intensities induce unfavourable feedbacks between nutrient cycling and resource allocation, constraining soil microbial activity, plant productivity, and drought resilience. In a native managed tropical grassland, four aboveground biomass removal frequencies (1×, 2×, 3×, and 6× cutting annually) were applied in the field. Intact soil core mesocosms were exposed to drought stress in a split-plot completely randomised design (CRD), and combined with 13CO2 pulse labelling. We assessed plant biomass yield, 13CO2 assimilation and belowground C allocation, arbuscular mycorrhizal fungi (AMF) colonisation, extracellular enzyme activities, and rhizosphere microbial community structure and function. Increased cutting frequency shifted vegetation from resource-conservative, “slow” strategists to more acquisitive, “fast” strategists, accompanied by a transition from oligotrophic, fungi-dominated to copiotrophic, bacteria-dominated rhizosphere communities. Drought reduced total 13C assimilation and biomass production but enhanced allocation of assimilates to symbiotic and stress-alleviating microbes, particularly AMF. Overall, our results demonstrate that biomass removal regulates plant–soil–microbe interactions underpinning ecosystem resilience and productivity under drought, highlighting the importance of optimising management intensity in tropical grasslands under climate change.

Understanding nitrogen (N) retention mechanisms in pristine humid temperate rainforest soils is critical for effective ecosystem management and nutrient conservation. The potential abiotic transformation of nitrite (NO₂⁻) into organic N forms in the absence of microbial activity in these ecosystems remains largely unexplored, despite its role in mitigating N leaching. This study focuses on the abiotic incorporation of nitrite (NO₂⁻) into dissolved organic nitrogen (DON) under anoxic conditions, a mechanistic step not directly evaluated in previous research, which employed ¹⁵N-labelled nitrate (NO₃⁻). To address this gap, we used ¹⁵N-labelled NO₂⁻ at 5 and 15 mg L⁻¹ in a lab incubation study under anoxic conditions to trace the contribution of abiotic nitrite transformation to organic N formation in organic matter-rich soils from temperate rainforests developed on both volcanic and non-volcanic parent materials. The added ¹⁵N declined rapidly after 15 min by 52% and 60% in both soil solutions, while it started to form labelled DON, increasing by 11% and 34%, after five days of incubation, with the highest accumulation at 15 mg L⁻¹ of ¹⁵N-NO₂⁻. These results show that up to 77% of the added ¹⁵N-NO₂⁻ can be abiotically incorporated into the DON of unpolluted old-growth temperate rainforest, whether developed on volcanic or non-volcanic soils. Nitrogen input has a stronger effect than soil parent material from which the soils originate. This reveals the natural resilience of unpolluted temperate rainforests to N loss, with implications for long-term ecosystem stability and nutrient cycling.

Plant litter decomposition is a key ecosystem process with significant implications for global carbon cycling, soil fertility and plant productivity. Given that microbial decomposers are the main players in the decomposition process, it is surprising how little is known about their functional diversity in different habitats or their ability to respond to environmental changes. To fill this knowledge gap, we conducted a litterbag decomposition experiment along a pronounced climate and vegetation gradient in the Chilean Coastal Cordillera (26°S to 38°S), ranging from hyper-arid to temperate, using mixtures of four plant species, native to the respective ecosystems. We analyzed potential decomposition functions of bacterial and fungal litter communities along with their biotic (litter traits) and abiotic (meteorological conditions and soil properties) environments, to determine the relative importance of these environmental factors for microbial community functioning. We also tested the impact of the functional diversity of the decomposer communities (i.e., the diversity of decomposition related functions) on litter mass loss. Functional composition was related most strongly to the temporal variation of precipitation and radiation, explaining about 19 % and 6 % of variation in bacteria and fungi, respectively. In contrast, functional diversity was quite strongly related to litter chemical traits (C:N, C:P, tannins, phenols). Litter mass loss after six months of decomposition was not correlated to the functional diversity of decomposer communities but increased with the presence of habitat generalists like Proteobacteria , Actinobacteria , Firmicutes , and Bacteroidetes . Taken together, these results highlight i) the interplay between abiotic factors and chemical litter traits on litter microbial functions and functional diversity and ii) the importance of microbial generalists for litter decomposition across different ecosystems. These results enhance our ability to predict changes in microbial decomposer communities and litter decomposition under future climate-change scenarios.

Arbuscular mycorrhizal fungi (AMF) are important partners in plant nutrition, as they increase the range to scavenge for nutrients and can access resources otherwise occlude for plants. Under water shortage, when mobility of nutrients in soil is limited, AMF are especially important to acquire resources and can modulate plant drought resistance. Strategies of plants to cope with water and nutrient restrictions are shaped by the intensity of aridity. To investigate the effect of aridity on plant-AMF associations regarding drought resistance and plant nutrient acquisition, a 13CO2 pulse labeling was conducted across an aridity gradient. In a semiarid shrubland (66 mm a-1), a Mediterranean woodland (367 mm a-1), and a humid temperate forest (1500 mm a-1), root and soil samples were taken from 0-10 cm and 20-30 cm soil depth before labeling and at 1 day, 3 days, and 14 days after labeling. Carbon (C), nitrogen (N), and phosphorus (P) stocks as well as AMF root colonization, extraradical AMF biomass (phospho- and neutral lipid fatty acids (PLFA and NLFA) 16:1w5c), specific root length (SRL), and root tissue density (RTD) were measured. Plant C investment into AMF and roots was determined by the 13C incorporation in 16:1w5c (PLFA and NLFA) and root tissue, respectively. Soil C:N:P stoichiometry indicated a N and P limitation under humid conditions and a P limitation in the topsoil under Mediterranean conditions. N stocks were highest in the Mediterranean woodland. A strong correlation of the AMF storage compound NLFA 16:1w5c to C:P ratio under semiarid conditions pointed to a P limitation of AMF, likely resulting from low P mobility in dry and alkaline soils. With increasing aridity, the AMF abundance in root (and soil) decreased from 45% to 20% root area. 13C incorporation in PLFA 16:1w5c was similar across sites, while relative AMF abundance in topsoil (PLFA 16:1w5c:SOC) was slightly higher under semiarid and humid than under Mediterranean conditions, pointing to the importance of AMF for plant nutrition under nutrient limitation. Additionally, PLFA 16:1w5c contents in soil were higher with lower P availability in each site, underlining the role of AMF to supply P for plants under P deficiency. Under humid conditions (with strong N and P limitation) and semiarid conditions (with strong water limitation), root AMF colonization increased with lower N availability, displaying the role of AMF for plant N nutrition under nutrient and/or water shortage. Under humid and Mediterranean conditions, SRL decreased (0.5 and 0.3 times, respectively) and RTD increased (1.9 and 1.7 times, respectively) with depth, indicating a drought tolerance strategy of plants to sustain water shortage. Under semiarid conditions, SRL increased with depth (2.3 times), while RTD was consistently high, suggesting an increasing proportion of long-living fine roots with depth as scavenging agents for water. These relations point to a drought avoidance strategy of plants as adaptation to long-term water limitation. Under strong nutrient limitation, as under humid and semiarid conditions, AMF are crucial to sustain plant nutrition and to enhance plant resistance to water shortage.

Temperate forests in Chile have experienced increasing temperatures and extreme climatic events, such as severe drought and short winters in unique Araucaria araucana forest in Nahuelbuta National Park. Therefore, it is relevant to understand the impact of drying and rewetting (D/R) or freezing and thawing (F/T) on SOM turnover in these ecosystems. Particularly important is the destabilization of soil organic matter (SOM) by microbial activity, which is highly heterogeneous and influenced by soil properties and water cycles. Drying and rewetting or F/T cycles accelerate particulate organic matter (POM) decomposition by aggregate disruption, thereby, decreasing carbon (C) availability for soil microorganism. We hypothesized that frequent D/R and F/T cycles release labile organic C locked away in the aggregates for microbial consumption. We assumed that a repeated number of D/R and F/T cycles enhance the preferential C utilization of fresh organic substrate. In the present study an incubation experiment was conducted for 27 days to assess the effect of F/T (-18 ºC to room temperature) and D/R (-500 kPa to 33 kPa, field capacity) cycles on labelled 14C glucose and 13C lignocellulose decomposition, soil aggregates size and POM fractions distributions. CO2 efluxes and priming effect (PE), i.e. the turnover acceleration or retardation of native C mineralization, C use efficiency (CUE) and C allocation in soil aggregate classes as POM-light, POM-occluded and heavy fractions were also determined. Labelled glucose was mainly allocated in macro (> 250 mm) and microaggregates (< 250 mm) as part of the POM-light fraction. In contrast, labelled lignocellulose was allocated in microaggregate in the POM-occluded and heavy fraction. CUE was similar amongst all treatments. The PE was negative in soil with and without cycles and it was much more pronounced (-125 mg C kg-1 soil) for F/T cycles than D/R (-50 mg C kg-1 soil) at the end of incubation. The C:N ratio of soil following mining theory is further discussed. We conclude that D/R cycles clearly retarded the native C mineralization by preferential use of labelled 13C-lignocellulose, while F/T cycles led to preferential use of 14C-glucose.

Besides nitrogen, phosphorus (P) is the major limiting nutrient of terrestrial primary productivity, with major P stocks being bound in soils. Stocks, speciation, and bioavailability of soil P differ among ecosystem types and with rock weathering status, which are both driven by climatic conditions. Microorganisms and plants have developed a range of strategies to mobilize P from organic and inorganic sources, e.g. expression of extracellular phosphatases and excretion of low-molecular-weight organic acids (LMWOA). However, the impact of precipitation, vegetation type, and soil P speciation on plant P acquisition strategies is not well understood, yet.A semi-desert-to-humid-temperate-rainforest ecosystem sequence was investigated. Soil samples were taken from three sampling sites, all developed on granodiorite, comprising a precipitation gradient (66 mm a-1 to 1469 mm a-1) along the Chilean Coastal Cordillera. Small-scale gradients (mm) from single roots to bulk soil in three depths were sampled to examine changes in P speciation, enabling the identification of local P depletion by plant roots and differences in P-speciation between rhizosphere and non-rhizosphere soil. Phosphorus speciation was examined by X-ray absorption near edge structure analysis. LMWOA as biotic weathering agents, and acid phosphatase kinetics as proxy for organic P recycling, were quantified. The aim was to disentangle the impact and functions of roots and associated microorganisms on driving agents of P-cycling.Rhizosphere P speciation in soil changed considerably along the precipitation gradient from mainly primary P minerals in the semi-desert ecosystem to a dominance of organic P species in the humid-temperate rainforest. Contents of organically bound P were higher in root proximity compared to bulk soil in the humid-temperate rainforest soils (320 mg kg-1 and 70 mg kg-1, respectively) and in the topsoil of the Mediterranean woodland ecosystem (134 mg kg-1  and 62 mg kg-1, respectively).  In contrast, the rhizosphere soil was depleted in sesquioxide-adsorbed P in comparison to root-free bulk soil.The content of LMWOA was correlated with inorganic P in soils of the semi-desert ecosystem, indicating intensive LMWOA exudation for biogenic P weathering of primary and secondary minerals. Under temperate rainforest LMWOA content, phosphatase activity, and microbial biomass carbon exhibited a negative correlation with secondary inorganic P forms but were positively linked to organic P species. We therefore conclude that P nutrition in this ecosystem relies less on weathering of P bearing minerals by LMWOA but is mainly based on organic P sources.In terms of process understanding, these findings clearly show that LMWOA fundamentally change their role in the rhizosphere depending on the nutrient acquisition strategy of the respective ecosystem, which is affected by mean annual precipitation. While LMWOA facilitate biogenic weathering of P bearing minerals in the semi-desert, they mainly contribute to P recycling in the humid-temperate rainforest by preventing its precipitation and sorption. We conclude that P acquisition and cycling depends on the nutritional constrains of the given ecosystem: from biological weathering of inorganic P forms in the semi-desert driven by LMWOA and plant uptake to intensive P recycling from organic forms in the humid-temperate rainforest.