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Hydrological cycle is expected to become the primary cause of ecosystem's degradation in near future under changing climate. Rain manipulation experiments under field conditions provide accurate picture on the responses of biotic processes to changed water availability for plants. A field experiment, mimicking expected changes in rain patterns, was established in a Mediterranean shrub community at Porto Conte, Italy, in 2001. In November 2011 Cistus monspeliensis, one of the dominating shrub species in the Mediterranean basin, was 13 C labelled on plots subjected to extended rain shortage period and on control non manipulated plots. Carbon (C) allocation was traced by 13 C dynamics in shoots, shoot-respired CO 2 , roots, microbial biomass, K 2 SO 4 -extractable C and CO 2 respired from soil. Most of the recovered 13 C (60%) was respired by shoots within 2weeks in control plots. In rain shortage treatment, 13 C remained incorporated in aboveground plant parts. Residence time of 13 C in leaves was longer under the rain shortage because less 13 C was lost by shoot respiration and because 13 C was re-allocated to leaves from woody tissues. The belowground C sink was weak (3-4% of recovered 13 C) and independent on rain manipulation. Extended rain shortage promoted C exudation into rhizosphere soil in expense of roots. Together with lowered photosynthesis, this "save" economy of new C metabolites reduces the growing season under rain shortage resulting in decrease of shrub cover and C losses from the system on the long-term., (Copyright © 2018. Published by Elsevier B.V.)

The study aimed to evaluate the effects of long‐term fertilisation on soil aggregation and the associated changes in soil organic carbon (SOC) and nitrogen (N) pools in aggregates. The combined application of mineral fertiliser and manure improved soil aggregation, SOC and N content in aggregates, compared to manure or mineral fertiliser alone, and thus proved to be a suitable fertilisation strategy to increase C sequestration in agroecosystems. [ABSTRACT FROM AUTHOR]

Decomposition of soil organic matter (SOM) protected within aggregates can be accelerated via priming effect (PE) by the addition of fresh substrates. However, the knowledge of the sources of mineralization and PE in aggregate size classes is absent. We applied the three-source-partitioning isotopic ( 14 C + δ 13 C) approach to determine how aggregate size classes affect the contribution of three C sources (substrate added, recent and old SOM) to CO 2 efflux and PE depending on the amount of added primer. Soil from a field with 3 years of maize cropping (C 4 plants) after long-term C 3 vegetation was used to differentiate between recent C (C 4 C; < 3 years) and old C (C 3 C; >3 years). Soil samples were separated into three aggregate size classes (>2 mm, 2–0.25 mm macroaggregates and <0.25 mm microaggregates) and were incubated for 49 days after being amended with two levels of 14 C labeled glucose. The proportion of glucose mineralized to CO 2 increased with decreasing aggregate size, but 14 C incorporation into microbial biomass decreased, indicating higher C use efficiency in macroaggregates compared with microaggregates. The short-time PE was positive and was accompanied by a rapid reduction of dissolved organic C. After 49 days, the PE was higher in macro-versus microaggregates at both glucose levels. Positive PE induced by a low glucose level was observed only in large macroaggregates (>2 mm), but was observed in both macroaggregates (>0.25 mm) and microaggregates (<0.25 mm) after high glucose amendment. These results indicate that SOM pools are more decomposable in macro-versus microaggregates and that the SOM pools are involved in PE according to their biochemical availability. More primed CO 2 originated from recent C 4 C than old C 3 C in larger macroaggregates under a low glucose level. The relative contribution of recent C 4 C to primed CO 2 increased from macroaggregates (37.8%) to microaggregates (100%) after high glucose amendment. Therefore, increasing glucose addition stimulated the decomposition of old C 3 C in macroaggregates, but not in microaggregates. This indicates that microaggregates protect SOM against decomposition better than macroaggregates, and consequently, microaggregates can be considered as a potential reservoir for long-term C sequestration. Concluding, aggregate size is crucial for SOM decomposition, and it determines the source of PE and thus the protection of sequestrated C. The effects of the added primer on C sources involved in PE depend on the aggregate size. [ABSTRACT FROM AUTHOR]