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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 effect of biochar and nitrapyrin (a nitrification inhibitor) applications on nitrous oxide (N2O) emissions from a cropland soil was studied in a 35-day incubation experiment. The biochars were produced using canola (Brassica napus L.) straw under two pyrolysis temperatures: 300 (BC300) and 700 °C (BC700). Biochars (20 g kg−1 soil) and nitrapyrin (80 mg kg−1 soil) were applied alone or in combination. The cumulative N2O emissions were affected by both biochar and nitrapyrin applications (p < 0.05, same below) but not by their interaction. Cumulative N2O emissions were not affected by BC700, but were increased by BC300, as compared with the CK treatment (no biochar addition). Nitrapyrin significantly decreased cumulative N2O emissions by inhibiting nitrification, whether biochar was applied or not. There were positive relationships (p < 0.05) between cumulative N2O emissions and soil microbial biomass carbon to nitrogen ratio, nitrate and dissolved organic nitrogen concentrations, and net nitrification rates. Our results show that biochars need to be appropriately selected (such as the use of BC700) that do not increase N2O emissions, while the effectiveness of nitrapyrin in reducing N2O emissions was not affected by the co-application of biochars. We conclude that the co-application of biochar and nitrapyrin may be able to both increase soil C sequestration by the addition of stable C contained in the biochar and reduce N2O emissions from agricultural production systems. [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]

How cattle urine and dung addition, and their interaction with the mineral soil regulate cumulative autotrophic and heterotrophic nitrifications and greenhouse gas (GHG) emission is poorly understood. We investigated the effect of urine (applied at 425 mg N kg−1 soil) and dung (applied at 200 mg total N kg−1 soil) addition on cumulative autotrophic and heterotrophic nitrifications, and GHG emission in a grassland soil in a 35-day laboratory incubation experiment under six treatments: CK, unamended control; U, urine addition; Ds, dung added on the soil surface; U-Ds, urine addition + dung added on the soil surface; Dm, dung mixed into the soil; and U-Dm, urine addition + dung mixed into the soil. Compared with the CK, the U, U-Ds, and U-Dm treatments increased cumulative net nitrification (autotrophic + heterotrophic) by 548, 587, and 505% and cumulative autotrophic nitrification by 593, 618, and 508%, but the Dm treatment decreased cumulative net and autotrophic nitrifications by 77 and 83%, respectively. However, cumulative heterotrophic nitrification was less than 1% of the cumulative autotrophic nitrification in the CK and it was decreased by Ds and Dm as compared with the CK at the end of the incubation. Cumulative emissions of carbon dioxide (CO2) and nitrous oxide (N2O) were in the order of CK < U < Ds < Dm < U-Ds < U-Dm. In addition, all urine and dung treatments reduced cumulative methane (CH4) uptake (negative values of CH4 emission) as compared with the CK. Overall, soil cumulative net and autotrophic nitrifications were higher, but GHG emission was lower, with dung added on the soil surface than with it mixed into the soil. We conclude that urine addition increases but dung addition decreases cumulative autotrophic nitrification; dung addition also decreases heterotrophic nitrification, and how dung is added to the soil is important in regulating the nitrification processes and GHG emission. Therefore, urine and dung management can alter soil N transformations and has implications for mitigating climate change. [ABSTRACT FROM AUTHOR]

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]

We evaluated the impact of exponential fertilization in nursery and weed removal in the field on growth and nitrogen (N) retranslocation and uptake from the soil of jack pine ( Pinus banksiana Lamb.) seedlings planted on an oil sands reclaimed soil. Exponential fertilization is a method of supplying nutrients at an exponential rate to achieve constant internal nutrient concentrations in seedlings without changing their size during their growth in the nursery. The N retranslocation in seedlings was traced using N isotope labeling. Exponential fertilization increased nutrient reserve in the seedling in nursery production, and increased height ( P = 0.003), root collar diameter ( P < 0.001), total biomass ( P < 0.001), and N content ( P < 0.001) of seedlings at the end of first growing season in the field growth. Conventionally fertilized seedlings allocated a greater percent of biomass to roots than to current-year needles. The N isotope analysis showed that 59 to 82% of total N demand of new growth was met by retranslocation from old tissues. Exponential fertilization increased N retranslocation by 147% ( P < 0.001) and N uptake from the soil by 175% ( P = 0.012). Weed removal marginally increased ( P = 0.077) N uptake from the soil but decreased ( P = 0.046) N retranslocation with no net effect on total N content in new tissues. We conclude that exponential fertilization improves the early growth of jack pine and can help improve revegetation in reclaiming disturbed oil sands sites. [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]