Despite the recognized importance of the contribution of microbial necromass to soil organic carbon (SOC) sequestration, at a global scale, there has been no quantification for cropland, grassland, and forest ecosystems. To address this knowledge gap, the contents of fungal and bacterial necromass were estimated based on glucosamine and muramic acid contents in cropland (986 samples), grassland (278 samples), and forest (452 samples) soils. On an average, microbial necromass C contributed 51%, 47%, and 35% to the SOC in cropland, grassland, and forest soils, respectively, in the first 20 cm of topsoil. The contribution of microbial necromass to SOC increased with soil depth in grasslands (from 47% to 54%) and forests (from 34% to 44%), while it decreased in croplands (from 51% to 24%). The microbial necromass accumulation coefficient (the ratio between necromass and living microbial biomass C) was higher in soil from croplands (41) and grasslands (33) than in forest (20) soils. These results suggest that the turnover of living microbial biomass is faster in grassland and cropland soils than in forest soils, where the latter contains more partially decomposed plant residues. Fungal necromass C (>65% of total necromass) had consistently higher contributions to SOC than bacterial necromass C (32–36%) in all soils due to i) a larger living fungal biomass than bacterial biomass, and ii) fungal cell compounds being decomposed slowly and, thus able to persist longer in soil. The ratio of fungal:bacterial necromass C increased from 2.4 to 2.9 in the order of croplands

Microbial necromass as the source of soil organic carbon in global ecosystems

Mechanisms of soil organic carbon (SOC) stabilization have been widely studied due to their relevance in the global carbon cycle. No-till (NT) has been frequently adopted to sequester SOC; however, limited information is available regarding whether sequestered SOC will be stabilized for long term. Thus, we reviewed the mechanisms affecting SOC stability in NT systems, including the priming effects (PE), molecular structure of SOC, aggregate protection, association with soil minerals, microbial properties, and environmental effects. Although a more steady-state molecular structure of SOC is observed in NT compared with conventional tillage (CT), SOC stability may depend more on physical and chemical protection. On average, NT improves macro-aggregation by 32.7%, and lowers SOC mineralization in macro-aggregates compared with CT. Chemical protection is also important due to the direct adsorption of organic molecules and the enhancement of aggregation by soil minerals. Higher microbial activity in NT could also produce binding agents to promote aggregation and the formation of metal-oxidant organic complexes. Thus, microbial residues could be stabilized in soils over the long term through their attachment to mineral surfaces and entrapment of aggregates under NT. On average, NT reduces SOC mineralization by 18.8% and PE intensities after fresh carbon inputs by 21.0% compared with CT (p < .05). Although higher temperature sensitivity (Q10 ) is observed in NT due to greater Q10 in macro-aggregates, an increase of soil moisture regime in NT could potentially constrain the improvement of Q10 . This review improves process-based understanding of the physical and chemical mechanism of protection that can act, independently or interactively, to enhance SOC preservation. It is concluded that SOC sequestered in NT systems is likely to be stabilized over the long term.

Mechanisms of soil organic carbon stability and its response to no-till: A global synthesis and perspective

Soil ecoenzymatic stoichiometry reveals microbial phosphorus limitation after vegetation restoration on the Loess Plateau, China

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Above-belowground interactions in alpine ecosystems on the roof of the world

Anaerobic digestate is a waste product of biogas generation which is produced in large amounts and, because of its high water content, it is expensive to store, transport, and spread to land. Additionally, special conditions are required for land application to minimise the losses of nutrients. This material is primarily used as source of organic matter for soil while farmers continue to rely on NPK chemical fertilisers, which are produced using energy intensive processes. This work evaluated the use of low-pollutant biomass bottom ash as an adsorbent to decrease the availability and the losses of carbon and nutrients. A number of acidification conditions were tested to enhance the adsorption and to improve the dewaterability of the organic waste. The final blend was intended to have a more balanced nutrient profile and to offer better performance in terms of crop growth than the digestate alone. The severe acidifications of the digestate and ash using sulphuric, hydrochloric, nitric, and lactic acids increased more than twice the amount of ammonia which remained in the digestate-ash blend. Hydrochloric acid was found to be the best option for preparation of the ash as sorbent, before mixing with the digestate, and to promote dehydration of the blend to enhance solid-liquid separation. This acid did not reduce the number of active sites in the ash, to promote the chemical stabilisation of the digestate; the addition of the acidified ash reduced the pH below that of the digestate thereby reducing the volatilisation of NH3 from the blend.

Impact of sulphuric, hydrochloric, nitric, and lactic acids in the preparation of a blend of agro-industrial digestate and wood ash to produce a novel fertiliser

Soil nutrient contents and organic carbon (C) stability are key indicators for restoration of degraded grassland. However, the effects of long-term active restoration of extremely degraded grassland on soil parameters have been equivocal. The aims of this study were to evaluate the impact of active restoration of degraded alpine grassland on: (a) soil organic matter (SOM) mineralization; and (b) the importance of biotic factors for temperature sensitivity (Q(10)) of SOM mineralization. Soils were sampled from intact, degraded and restored alpine grasslands at altitudes ranging between 3,900 and 4,200 m on the Tibetan Plateau. The samples were incubated at 5, 15 and 25 degrees C, andQ(10)values of SOM mineralization were determined. Structural equation modeling was used to evaluate the importance of vegetation, soil physico-chemical properties and microbial parameters forQ(10)regulation. TheQ(10)of N mineralization was similar among intact, degraded and restored soils (0.84-1.24) and was higher in topsoil (1.09) than in subsoil (0.92). The best predictive factor of CO2-Q(10)for intact grassland was microbial biomass, for degraded grassland was basal microbial respiration, and for restored grassland was soil bulk density. Restoration by planting vegetation decreased theQ(10)of SOM mineralization as soil bulk density, the most important negative predictor, increased in restored grassland. TheQ(10)of SOM mineralization in topsoil was 14% higher than in subsoil because of higher microbial abundance and exo-enzyme activities. The NH(4)(+)content was greatest in intact soil, while NO(3)(-)content was greatest in degraded soil. The SOM mineralization rate decreased with grassland degradation and increased after long-term (>10 years) restoration. In conclusion, extremely degraded grassland needs proper long-term management in active restoration projects, especially for improvement of soil nutrients in a harsh environment.

Lina Ma

Papers

Long‐term active restoration of extremely degraded alpine grassland accelerated turnover and increased stability of soil carbon