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Abstract The effect of warming on soil nitrogen (N) pools and turnover in agro-ecosystems has rarely been investigated, particularly in middle-high latitudes where the temperature is predicted to increase more than that in low latitudes. In this study, we determined the dynamics of soil inorganic N pools and the net N mineralization in an agro-ecosystem from 2013 to 2015 in a field warming experiment in Northeast China. The objectives were to examine the effects of warming on soil N turnover and determine how the effects differ in the growing versus the non-growing seasons. We hypothesized that experimental warming would increase N mineralization and the inorganic N pools, and the effects of warming would be greater in the non-growing season than those in the growing season. In contrast to these predictions, the soil N mineralization and inorganic N pools in the soil solution were not affected by experimental warming either within or across year and season. The nitrification rate was 0.14 mg kg−1 d−1 higher, but the ammonification rate was 0.17 mg kg−1 d−1 lower in the growing season than that in the non-growing season. When averaged across all sources of variation, the net mineralization rate of the soil N was 0.09 mg kg−1 d−1 in both the control and warming treatments, while the total inorganic N in the soil solution was 34 and 41 mg kg−1, respectively. Both the nitrification and ammonification processes were regulated by soil moisture and/or root biomass, which was not affected by warming. These results suggest that soil N turnover and availability will be less sensitive to future global warming in this middle-high latitude agro-ecosystem. Graphical abstract Unlabelled Image Highlights • Warming did not affect soil N mineralization and inorganic N pool. • Warming had no impact on nitrification and ammonification processes. • The growing season had a higher Rn, but lower Ra than the non-growing season. [ABSTRACT FROM AUTHOR]

Abstract: Toxic effects of aluminium (Al) on Picea abies (L.) Karst. (Norway spruce) trees are well documented in laboratory-scale experiments, but field-based evidence is scarce. This paper presents results on fine root growth and chemistry from a field manipulation experiment in a P. abies stand that was 45 years old when the experiment started in 1996. Different amounts of dissolved aluminium were added as AlCl3 by means of periodic irrigation during the growing season in the period 1997–2002. Potentially toxic concentrations of Al in the soil solution were obtained. Fine roots were studied from direct cores (1996) and sequential root ingrowth cores (1999, 2001, 2002) in the mineral soil (0–40 cm). We tested two hypotheses: (1) elevated concentration of Al in the root zone leads to significant changes in root biomass, partitioning into fine, coarse, living or dead fractions, and distribution with depth; (2) elevated Al concentration leads to a noticeable uptake of Al and reduced uptake of Ca and Mg; this results in Ca and Mg depletion in roots. Hypothesis 1 was only marginally supported, as just a few significant treatment effects on biomass were found. Hypothesis 2 was supported in part; Al addition led to increased root concentrations of Al in 1999 and 2002 and reduced Mg/Al in 1999. Comparison of roots from subsequent root samplings showed a decrease in Al and S over time. The results illustrated that 7 years of elevated Altot concentrations in the soil solution up to 200 μM are not likely to affect root growth. We also discuss possible improvements of the experimental approach. [Copyright &y& Elsevier]

Overseeding legumes in natural grasslands coupled with phosphorous fertilization are management practices oriented to increase forage production and quality, and to restore nutrient losses generated by livestock. Several studies show increases in forage due to this practice, but less is known about impacts on soil fertility and carbon sequestration. The objective of this study was to evaluate under real farm conditions changes in root C and N stocks and soil organic carbon (SOC) and nitrogen (SON) stocks in two different soil pools, the particulate organic matter (POM) and the mineral associated organic matter (MAOM), after the introduction in natural grasslands of a legume species, Lotus subbiflorus cv. "El Rincón", accompanied with phosphorous fertilization. We also evaluated changes in the natural abundance of 15N and 13C in soils and roots to understand changes in N fixation and species composition. We selected 12 adjacent paddocks of natural grasslands (NG) and natural grasslands overseeded with legumes and fertilized with phosphorous (NGLP) located in commercial farms in Uruguay. We found that overseeding legumes increased root C and N stocks and SOC and SON stocks in some farms but decreased them in others. On average, no significant differences arose between NGLP and NG paddocks in total stocks of 0–30 cm depth. However, higher C stocks were observed in POM of NGLP paddocks in 0–5 cm layer and lower contents in 5–10 cm layer indicating a change in the vertical distribution of C in POM. Changes in δ15N suggest that atmospheric N is being fixed by legumes in NGLP paddocks, but not translated into more N or C stocks in the MAOM fraction, probably due to high N losses promoted by cattle grazing. Our work suggests that carbon sequestration can be achieved after legumes introduction in grazed natural grasslands but will depend on grazing management practices. Unlabelled Image • Overseeding legumes in grasslands is a usual practice to increase forage production. • Overseeding legumes and P fertilization in grasslands may increase N fixation. • Both practices are expected to increase root biomass and soil organic matter. • On-farm surveys showed both increases and decreases of roots and soil organic matter. • Observed opposite changes are suspected to depend on farm's management. [ABSTRACT FROM AUTHOR]

Ground-level ozone (O 3) pollution often co-occurs with anthropogenic nitrogen (N) deposition. Many studies have explored how O 3 and soil N affect aboveground structure and function of trees, but it remains unclear how belowground processes change over a spectrum of N addition and O 3 concentrations levels. Here, we explored the interactive impact of O 3 (five levels) and soil N (four levels) on fine and coarse root biomass and biomass allocation pattern in poplar clone 107 (Populus euramericana cv. '74/76'). We then evaluated the modifying effects of N on the responses of tree root biomass to O 3 via a synthesis of published literature. Elevated O 3 inhibited while N addition stimulated root biomass, with more pronounced effects on fine roots than on coarse root. The root:shoot (R:S) ratio was markedly decreased by N addition but remained unaffected by O 3. No interactive effects between O 3 and N were observed on root biomass and R:S ratio. The slope of log–log linear relationship between shoot and root biomass (i.e. scaling exponent) was increased by N, but not significantly affected by O 3. The analysis of published literature further revealed that the O 3 -induced reduction in tree root biomass was not modified by soil N. The results suggest that higher N addition levels enhance faster allocation of shoot biomass while shoot biomass scales isometrically with root biomass across multiple O 3 levels. N addition does not markedly alter the sensitivity of root biomass of trees to O 3. These findings highlight that the biomass allocation exhibits a differential response to environmentally realistic levels of O 3 and N, and provide an important perspective for understanding and predicting net primary productivity and carbon dynamics in O 3 -polluted and N-enriched environments. Unlabelled Image • Root biomass and biomass allocation were studied for poplar at 5 O 3 and 4 N levels. • Elevated O 3 inhibited but N addition stimulated root biomass. • R:S ratio was decreased by N addition but not affected by elevated O 3. • No interactive effects of O 3 and N on root and R:S ratio. • Soil N addition did not ameliorate O 3 -induced reductions in root biomass. [ABSTRACT FROM AUTHOR]