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Terrestrial ecosystems have experienced rising nitrogen (N) inputs during the last decades with consequences for belowground carbon (C) and N dynamics. This study investigates how long-term N and phosphorus (P) additions affect microbial community composition, and to what extent microbial homeostasis explains changes in different processes involved in soil C and N cycling in response to nutrient addition. We studied a 66-year-old nutrient addition experiment in a mesic grassland in South Africa, consisting of four different levels of N addition (0, 7, 14, and 21 g N m−2 yr−1) with and without P addition (0, and 9 g P m−2 yr−1). Despite strong changes in the microbial community (observed through 16S rRNA gene and ITS amplicon sequencing), the microbial biomass C:N ratio did not change. N addition decreased microbial N acquisition as indicated by reduced leucine-aminopeptidase activity, and increased microbial net N mineralization. In contrast, predicted relative abundances of functional genes involved in degradation of labile C compounds (e.g. cellulose, hemicellulose, and chitin) as well as β-glucosidase and N-acetylglucosaminidase activities increased with elevated N availability. In combination, this pointed to a more intensive investment of microorganisms into C acquisition upon N addition. In contrast, N addition and associated soil acidification decreased microbial biomass and respiration and altered the community composition with prokaryotes being more affected than fungi. Nitrogen addition increased the relative abundance of gram-positive over gram-negative bacteria and favored taxa with low genome-size. Taken together, our findings support the concept that C and N cycling processes can be explained by the property of the soil microbial community to keep the element ratio of its biomass constant and by its reaction to soil acidification. Our findings suggest that predicted elevated N inputs might largely shape soil C and N cycling because the soil microbial community adjusts metabolic processes, which allows it to maintain its biomass stoichiometry constant.

Addition of inorganic phosphorus (P) to soil has often been reported to cause increases in soil respiration, and this has been attributed to an alleviation of microbial P limitation. As an alternative explanation for this phenomenon, we tested the hypothesis that addition of inorganic P increases microbial respiration because added inorganic P exchanges with sorbed organic compounds in soil, and thus renders these organic compounds available for microbial decomposition. We conducted an experiment with 14C-labeled adenosine-monophosphate (AMP), and we determined the effect of inorganic P addition on dissolved organic carbon (DOC), dissolved DNA and soil respiration in the organic horizon and the A horizon of two beech forest soils with contrasting P stocks. We added inorganic P to the four soil horizons that contained trace amounts of 14C-AMP, and found that the emission of 14C-CO2 increased significantly due to P addition in all soil horizons by a factor of 1.4 to 4.0. Respiration rates increased significantly in all soil horizons by a factor 1.1 to 1.9 due to inorganic P addition. One hour after the addition of inorganic P, DOC concentrations were increased by a factor of 1.6 to 3.5 compared to the controls, and they were still similarly high seven days after the addition of inorganic P. Furthermore, concentrations of dissolved, extracellular DNA were also significantly increased in response to P addition. The 14C experiment shows that the addition of inorganic P led to increased microbial metabolization of a compound that strongly sorbs to the soil solid phase and suggests that the source of the additionally respired C is dead organic matter that desorbs upon inorganic P addition. The extraction experiments show that addition of inorganic P led to elevated DOC concentrations. In conclusion, our study supports the hypothesis that the addition of inorganic P increases microbial respiration because added inorganic P exchanges with sorbed organic compounds in soil, and thus turns these compounds available for microbial decomposition. The results indicate that microbial respiration was increased in response to P addition not because microbial P limitation was alleviated but because microbial C limitation was alleviated by desorbed organic C.

Kobresia grasslands on the Tibetan Plateau comprise the world’s largest pastoral alpine ecosystem. Overgrazing-driven degradation strongly proceeded on this vulnerable grassland, but the mechanisms behind are still unclear. Plants must balance the costs of releasing C to soil against the benefits of accelerated microbial nutrient mineralization, which increases their availability for root uptake. To achieve the effect of grazing on this C-N exchange mechanism, a 15NH4+ field labeling experiment was implemented at grazed and ungrazed sites, with additional treatments of clipping and shading to reduce belowground C input by manipulating photosynthesis. Grazing reduced gross N mineralization rates by 18.7%, similar to shading and clipping. This indicates that shoot removal by grazing decreased belowground C input, thereby suppressing microbial N mining and overall soil N availability. Nevertheless, NH4+ uptake rate by plants at the grazed site was 1.4 times higher than at the ungrazed site, because plants increased N acquisition to meet the high N demands of shoot regrowth (compensatory growth: grazed > ungrazed). To enable efficient N uptake and regrowth, Kobresia plants have developed specific traits (i.e., efficient above-belowground interactions). These traits reflect important mechanisms of resilience and ecosystem stability under long-term moderate grazing in an N-limited environment. However, excessive (over)grazing might imbalance such C-N exchange and amplify plant N limitation, hampering productivity and pasture recovery over the long term. In this context, a reduction in grazing pressure provides a sustainable way to maintain soil fertility, C sequestration, efficient nutrient recycling, and overall ecosystem stability.

The Tibetan Plateau hosts the world’s largest alpine pastoral ecosystems, dominated by the endemic sedges Kobresia pygmaea and Kobresia humilis. Owing to the very harsh environment and also to soil nitrogen (N) and phosphorus (P) limitations, these pastoral ecosystems are very sensitive to disturbances (e.g. anthropogenic activities and climate change) and recover extremely slowly. Overgrazing on the Tibetan Plateau has caused severe degradation of vegetation and soils in the last 30–50 years. For the first time, for Kobresia pastures in Tibetan Plateau, we have summarized and generalized the consequences of pasture degradation for soil organic carbon (SOC) and nutrient (N, P) stocks, and evaluated the main biotic and abiotic mechanisms of their loss. Based on 44 literature studies as well as own data, we demonstrated that 42% of SOC stocks were lost, relative to non-degraded pastures. These SOC losses are similar to the decreases in N stocks (-33%), and aboveground (-42%) and belowground (-45%) plant biomass. Although P losses are lower (-17%), its precipitation reduces its availability for plants. These losses are in fact underestimates, since undisturbed natural sites no longer exist on the Tibetan Plateau. The losses are much higher in the upper 10 cm and in some areas extend to complete removal of soil cover. This has dramatic repercussions for local livestock, human populations and river pollution. While some rehabilitation projects have shown positive outcomes, the complete recovery of degraded pastures (e.g. soil fertility, ecosystem stability) is infeasible, because of very slow pedogenic processes, slow vegetation restoration, as well as continuously increasing anthropogenic pressure and climate change. Considering the rapid losses of SOC and nutrients, and the very slow recovery potential, Tibetan pastures in some regions may disappear in the next few decades without proper and effective recovery strategies.

Kobresia pastures on the Tibetan Plateau are the largest alpine pastoral ecosystems. Kobresia pastures have experienced severe degradation in recent decades, inducing large nitrogen (N) losses from these ecosystems. This is particularly problematic, as it intensifies prevailing N limitation in these regions. Simultaneously, anthropogenic N deposition has increased across these ecosystems, but the fate of added N on variously degraded Kobresia pastures remains unclear. Kobresia pastures of three degradation stages were investigated: living, dying and dead root mats. High and very low (as a tracer) amounts of ¹⁵N‐labelled ammonium nitrate (NH₄NO₃) were applied to root mats under controlled conditions. Leaching was simulated over 3 months, and ¹⁵N recovery was measured in the plant–soil system. N addition promoted aboveground biomass and foliar N content of Kobresia during the early growth period, indicating a short‐term offset of N limitation. After 7–8 weeks, plant growth and ¹⁵N uptake were reduced in plants with initial N addition, reflecting a transition to N limitation induced by N uptake and leaching from soil. This limitation was also indicated by the strong decline of NO₃⁻ in leachates from living root mats compared with degraded root mats. Leaching N losses from dying and dead root mats increased 2⋅2 and 6⋅3 times, respectively, compared with those of living root mats. We conclude that N addition can facilitate plant growth in living root mats but contributes to N leaching in degraded pastures. This contribution to N leaching may weaken ecosystem recovery, increase NO₃⁻ loading of adjacent lower landscape parts and cause eutrophication of aquatic ecosystems. Copyright © 2017 John Wiley & Sons, Ltd.

Human activities have increased nitrogen (N) and phosphorus (P) inputs in terrestrial ecosystems and altered carbon (C) availability, shifting the stoichiometry of microbial substrates in soils, such as the C:N:P ratios of the dissolved organic matter pool. These stoichiometric deviations between microbial biomass and its substrate may control microbial processes of N cycling. We studied the effects of this stoichiometric mismatch using a full factorial N and P addition experiment replicated in six grassland ecosystems in South Africa, the USA, and the UK. We found that N and P addition changed the dissolved organic matter C:N ratio, but not the C:N ratio of the soil microbial biomass. Compared to P addition, N addition decreased microbial N acquisition via non-symbiotic N2 fixation by −55% and increased microbial N release via net N mineralization by +134%. A possible explanation is that the dissolved elements, e.g., dissolved organic C (DOC) and dissolved total N (DN), serve as the main microbial substrate and its C:N ratio defines whether N is scarce or abundant with respect to microbial demands. If N is available in excess relative to microbial demands, net N mineralization increases. In contrast, when N is scarce, immobilization outweighs release decreasing net N mineralization. However, the activity of leucine aminopeptidases, which decompose peptides, was not affected by nutrient additions. Further, C rather than P availability may control the rates of non-symbiotic N2 fixation in the six studied grassland sites. In conclusion, globally increasing nutrient inputs change processes of microbial N acquisition and release in grassland ecosystems and these changes are largely driven by shifts in substrate stoichiometry.

A widespread pattern of the plateau is mosaics of grasslands of Cyperaceae and grasses with forbs, interspersed with patches covered by lichen crusts induced by overgrazing. However, the fate of inorganic and organic N in non-crusted and crusted patches remains unknown in Kobresia grasslands. We reported on a field 15N-labeling experiment in two contrasting patches to compare retention of organic and inorganic N over a period of 29 days. 15N as KNO3, (NH4)2SO4 or glycine was sprayed onto their soil surface. We found that crusted patches decreased plant and soil N stocks. More 15N from three N forms was recovered in soil in both patches 29 days after the labeling. In non-crusted patches, 15N recovery by the living roots was about two times higher than in crusted ones, mainly due to higher root biomass. Microorganisms in non-crusted patches were N-limited due to more living roots and competed strongly for N with roots. Inorganic N input to these patches could alleviate N limitation to plants and microorganisms, and leads to higher total 15N recovery (plant + soil) for inorganic N forms. Compared to non-crusted patches, microorganisms in crusted patches were more C-limited due to depletion of available C caused by less root exudation. Added glycine could activate microorganisms, together with the hydrophobicity of glycine and crusts, leading to higher 15N-glycine than inorganic N. We conclude that crusts in Kobresia grasslands changed the fate of inorganic and organic N, and lead to lower total recovery from inorganic N but higher from organic N. This article is protected by copyright. All rights reserved.

Microorganisms mediate nutrient cycling in soils, and thus it is assumed that they largely control responses of terrestrial ecosystems to anthropogenic nutrient inputs. Therefore, it is important to understand how increased nitrogen (N) and phosphorus (P) availabilities, first, affect soil prokaryotic and fungal community composition and second, if and how changes in the community composition affect soil element cycling. We measured soil microbial communities and soil element cycling processes along a nine-year old experimental N-addition gradient partially crossed with a P-addition treatment in a temperate grassland. Nitrogen addition affected microbial community composition, and prokaryotic communities were less sensitive to N addition than fungal communities. P addition only marginally affected microbial community composition, indicating that P is less selective than N for microbial taxa in this grassland. Soil pH and total organic carbon (C) concentration were the main factors associated with prokaryotic community composition, while the dissolved organic C-to-dissolved N ratio was the predominant driver of fungal community composition. Against our expectation, plant biomass and plant community structure only explained a small proportion of the microbial community composition. Although microbial community composition changed with nutrient addition, microbial biomass concentrations and respiration rates did not change, indicating functional redundancy of the microbial community. Microbial respiration, net N mineralization, and non-symbiotic N2 fixation were more strongly controlled by abiotic factors than by plant biomass, plant community structure or microbial community, showing that community shifts under increasing nutrient inputs may not necessarily be reflected in element cycling rates. This study suggests that atmospheric N deposition may impact the composition of fungi more than of prokaryotes and that nutrient inputs act directly on element-cycling rates as opposed to being mediated through shifts in plant or microbial community composition.