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Rice (Oryza sativa L.) residues (root and straw) are unique substrates for microbes in paddy soils. Microbial respiration (RH) of paddy soils is likely to be affected by substrate quality. However, no relevant study on the effects of changed rice residue chemistry by climate change on the temperature sensitivity (Q10) of the RH of soils amended with the residues is available. To fill this knowledge gap, we conducted an incubation experiment (at 15, 25, and 35 °C) with rice residues of which lignin, nonstructural carbohydrate (NSC), and carbon-to-nitrogen ratio (C/N) were modified by growing the rice plants under different CO2 and air temperature (Tair). Co-elevated CO2 and Tair decreased lignin and C/N but increased the NSC contents of the residues, leading to the greater RH in the soils amended with rice residues produced under co-elevated CO2 and Tair compared to other CO2 or Tair treatments. However, the Q10 values of the RH of soils amended with rice residues produced under different CO2 and Tair were marginally changed, probably due to other confounding factors such as soil enzyme activity and nutrient availability. Despite that, there was a negative relationship between NSC and Q10 for soils amended with straw of which C/N is lower than root, agreeing with the kinetic theory on "substrate quality–Q10 relationship". Our result suggests that residue chemistry may not affect the Q10 of soils with root residue, but for straw-amended soils, less-recalcitrant NSC rather than more-recalcitrant lignin of straw may be critical in determining the Q10 of the soils. [ABSTRACT FROM AUTHOR]

This review analyzes the data on co-variations in δ13C and δ15N of soils with land-use types, management, and disturbance obtained from literature to explore potential implications of the dual isotopes in the study of soil organic matter (SOM) sources and C and N processes. Overall, croplands (δ13C and δ15N were − 20.3 ± 4.4‰ and + 7.6 ± 4.4‰, respectively) had greater isotopic values than grasslands (‒26.3 ± 3.0‰ and + 5.4 ± 1.1‰, respectively) and forests (− 26.0 ± 1.1‰ and + 4.3 ± 2.2‰, respectively). For intensively managed lands such as croplands and grasslands, application of organic inputs such as manure and compost of which isotopic signatures differed from the indigenous SOM was the main driver of co-variations in the δ13C and δ15N of SOM. For natural forests, both δ13C and δ15N of SOM co-increased with soil depth, reflecting heavy isotope enrichment during microbial stabilization of SOM and the potential influence of 13C-depleted atmospheric CO2 and 15N-depleted N deposition on the upper soils. Such vertical co-enrichments of 13C and 15N were disturbed by a land-use conversion to other lands including croplands. Though there were indications that land management practices such as tillage in croplands and grazing in grasslands, land-use changes, and land disturbance including forest fire might also affect both δ13C and δ15N, more data need to be accumulated to find a general trend of the isotopic variations of SOM. Analysis of both δ13C and δ15N may enlarge understanding of changes in SOM sources and soil C and N cycling by land-use types, management, change, and disturbance. [ABSTRACT FROM AUTHOR]

The temperature sensitivity (Q10) of heterotrophic soil respiration (RH) of forests affects terrestrial carbon (C) dynamics under climate change. However, the effects of changed chemistry of tree leaf litter (referred to as litter hereafter) under elevated atmospheric CO2 concentrations ([CO2]) on the Q10 of RH has not been explored. To fill the knowledge gap, the Q10 of RH was investigated through incubation experiments at three temperature levels using litters of pine (Pinus densiflora Siebold & Zucc.) and oak (Quercus variabilis Blume) that were produced under ambient ([CO2]a) and elevated ([CO2]e) [CO2]. Regardless of [CO2] conditions, pine litter had higher lignin but lower nonstructural carbohydrate (NSC), calcium (Ca), and manganese (Mn) concentrations than oak, suggesting that pine litter has inherently poor chemistry. [CO2]e increased the ratios of lignin-to-nitrogen (lignin/N; by 58 and 122% for pine and oak, respectively) and C-to-N (C/N; by 47 and 125%, respectively) of litter through decreasing litter N concentration. For soils amended with [CO2]a-litter, Q10 was greater for pine (1.54) than oak (1.39) litters in agreement with the kinetic theory on the substrate quality-Q10 relationship. However, amending [CO2]e-litter decreased Q10 for pine (1.44) but marginally increased Q10 for oak (1.44). The decreased Q10 for pine by [CO2]e could be attributed to the inherently poor litter chemistry (e.g., high lignin, and low NSC, Ca, and Mn concentrations). Our study shows that the changed litter chemistry by [CO2]e modifies Q10 of RH of litter-amended soils. [ABSTRACT FROM AUTHOR]

Rice residue including root and straw are unique carbon (C) source in paddy soils. However, the potential changes in quantity and chemical composition of rice residue under co-elevated atmospheric CO2 concentration ([CO2]) and air temperature (Tair) and the legacy effect of the changed chemical composition on residue decomposition have not been investigated. This study was conducted to investigate biomass, chemical composition, and decomposability of rice root and straw produced under elevated [CO2] and Tair. Root and straw biomass increased by elevated [CO2] and elevated Tair, respectively, and the greatest biomass was achieved under co-elevated [CO2]-Tair for both root and straw. The concentration of lignin (recalcitrant) decreased while that of nonstructural carbohydrates (less recalcitrant) increased by co-elevated [CO2]-Tair. The ratio of lignin-to-nitrogen (lignin/N) decreased by co-elevated [CO2]-Tair compared to ambient [CO2]-Tair due to increased N and decreased lignin concentrations. Decomposability of root (lignin/N, 36.4) produced under co-elevated [CO2]-Tair was greater than that under ambient co-elevated [CO2]-Tair (lignin/N, 53.7); however, there was no difference in decomposability for straw, which had relatively narrow range of lignin/N (27.3–36.5) regardless of [CO2]-Tair conditions. The results of this study provide a novel insight into the changes in quantity and quality of rice residue under elevated [CO2]-Tair that are necessary to predict changes in paddy soil C sequestration under global warming. [ABSTRACT FROM AUTHOR]

Using the stable N isotope technique, it is possible to trace N derived from many sources and pools and thus quantify the gross N transformation rates and plant N uptake, and the processes of gaseous N emissions (Choi et al. [2]). Carbon (C) and nitrogen (N) are key elements governing soil productivity and sustainability in terrestrial ecosystems including croplands, forests, and wetlands. The synthesized and experimental data published in the articles of this issue provides novel insights into the use of stable C and N isotopes in the study of C and N dynamics in different land uses under global change. [Extracted from the article]

Elevated atmospheric CO2 concentration ([CO2]) may change litter chemistry which affects litter decomposability. This study investigated respiration and microbial biomass of soils amended with litter of Pinus densiflora (a coniferous species; pine) and Quercus variabilis (a deciduous species; oak) that were grown under different atmospheric [CO2] and thus had different chemistry. Elevated [CO2] increased lignin/N through increased lignin concentration and decreased N concentration. The CO2 emission from the soils amended with litter produced under the same [CO2] regime was greater for oak than pine litter, confirming that broadleaf litter with lower lignin decomposes faster than needle leaf litter. Within each species, however, soils amended with high lignin/N litter grown under elevated [CO2] emitted more CO2 than those with low lignin/N litter grown under ambient [CO2]. Such contrasting effects of lignin/N on inter- and intra-species variations in litter decomposition should be ascribed to the effects of other litter chemistry variables including nonstructural carbohydrate, calcium and manganese as well as inhibitory effect of N on lignin decomposition. The microbial biomass was also higher in the soils amended with high lignin/N litter than those with low lignin/N litter probably due to low substrate use efficiency of lignin by microbes. Our study suggests that elevated [CO2] increases lignin/N for both species, but increased lignin/N does not always reduce soil respiration and microbial biomass. Further study investigating a variety of tree species is required for more comprehensive understanding of inter- and intra-species variations of litter decomposition under elevated [CO2]. [ABSTRACT FROM AUTHOR]

Waterlogged paddy and water-unsaturated upland ecosystems have contrasting soil nitrogen (N) processes that affect the natural N abundance (N/N, expressed as δN) in different N pools. In this study, we investigated the δN patterns in soil and plant N pools of paddy and upland ecosystems. Samples were collected from 20 each of paddy and upland fields at the active growing season and analyzed for N concentration and δN. The higher ( P < 0.001) concentration (22.1 mg N kg) and lower δN (6.9 ‰) of NH in paddy than in upland soils (6.1 mg N kg and 9.2 ‰, respectively) likely reflected the lower nitrification potential in paddy soils. On the other hand, a higher ( P < 0.001) δN of NO in paddy (12.7 ‰) than in upland soils (4.7 ‰) indicated higher denitrification rates in paddy soils. The positive ( P < 0.01) correlation of the δN of soil organic N with the δN of NH rather than with that of NO suggested a tight linkage between organic N and NH as a result of immobilization-mineralization turnover of NH. Therefore, in waterlogged paddy soils where nitrification was restricted, a high rate of microbial immobilization-mineralization turnover might lead to a lower δN of soil N (total N, organic N, and NH) than those for upland. The δN in plant N pools also reflected the dominant mineral N species in paddy (NH) and upland soils (NO). We conclude that contrasting soil N processes (particularly nitrification) leave distinct δN signatures in the soil and plant N pools between paddy and upland fields. [ABSTRACT FROM AUTHOR]

Nitrogen isotope abundance ( δN) of paddy rice ( Oryza sativa L.) grown for 110 days after transplanting (DAT) under field conditions with ammonium sulfate (AS with −0.4‰ as a synthetic fertilizer), pig manure compost (PMC with 15.3‰ as a livestock manure compost), and hairy vetch (HV with −0.5‰ as a green manure) was investigated to test the possible use of δN technique in discriminating organically grown from conventionally grown rice. At 15 DAT, the δN of whole rice decreased ( P < 0.05) in the order of 10.5‰ for PMC > 5.5‰ for control (without N input) > 4.0‰ for HV > 1.8‰ for AS. This difference seemed to reflect primarily the δN signal of N sources. Although differences in δN of rice grown with isotopically distinct N inputs (i.e. PMC vs. AS and PMC vs. HV) became smaller over time, the difference (2.8 and 3.0‰ difference at harvest on 110 DAT, respectively) was still significant ( P < 0.05). However, there was no distinguishable difference between AS and HV treatment after 42 DAT. Such effect of N inputs on δN of whole rice was also observed for root, shoot, and grain at harvest. Therefore, our study suggests that it is possible to distinguish rice grown with manure composts from that grown with synthetic fertilizers. However, if green manure of preceding N-fixing plants is used as the N source, δN of rice may not be a good surrogate of N sources. [ABSTRACT FROM AUTHOR]

The temperature sensitivity (Q10) of heterotrophic soil respiration (RH) of forests affects terrestrial carbon (C) dynamics under climate change. However, the effects of changed chemistry of tree leaf litter (referred to as litter hereafter) under elevated atmospheric CO2 concentrations ([CO2]) on the Q10 of RH has not been explored. To fill the knowledge gap, the Q10 of RH was investigated through incubation experiments at three temperature levels using litters of pine (Pinus densiflora Siebold & Zucc.) and oak (Quercus variabilis Blume) that were produced under ambient ([CO2]a) and elevated ([CO2]e) [CO2]. Regardless of [CO2] conditions, pine litter had higher lignin but lower nonstructural carbohydrate (NSC), calcium (Ca), and manganese (Mn) concentrations than oak, suggesting that pine litter has inherently poor chemistry. [CO2]e increased the ratios of lignin-to-nitrogen (lignin/N; by 58 and 122% for pine and oak, respectively) and C-to-N (C/N; by 47 and 125%, respectively) of litter through decreasing litter N concentration. For soils amended with [CO2]a-litter, Q10 was greater for pine (1.54) than oak (1.39) litters in agreement with the kinetic theory on the substrate quality-Q10 relationship. However, amending [CO2]e-litter decreased Q10 for pine (1.44) but marginally increased Q10 for oak (1.44). The decreased Q10 for pine by [CO2]e could be attributed to the inherently poor litter chemistry (e.g., high lignin, and low NSC, Ca, and Mn concentrations). Our study shows that the changed litter chemistry by [CO2]e modifies Q10 of RH of litter-amended soils. [ABSTRACT FROM AUTHOR]