Description: Backgrounds and aims Soybean yield depends on total N uptake, N use efficiency, and harvest index. Nitrogen uptake relays on biological fixation (BNF) and soil absorption. Usually, BNF is considered a yield-related process. However, there is limited information on whether maximizing percent BNF (%BNF) is actually required to maximize N uptake and yield. Methods Seventy cultivars were evaluated for total N uptake, N use efficiency, and harvest index. Biological N fixation was determined in a subset of cultivars. The harvest index of N derived from atmosphere and from soil was also assessed. Results Yield was positively associated with total N uptake. Highest N uptake was not linked to increased %BNF. An inverse relationship between the amount of BNF (kgBNF) and soil N absorption was observed. Harvest index of N derived from BNF was 85%, while it was 77% for N derived from soil. Conclusions Highest total N uptake was attained by different combinations of kgBNF and mineral soil N absorption. This showed that maximizing %BNF is not required to maximize yield. High %BNF played a pivotal role in determining neutral soil N balance. This is so even though N derived from BNF was more partitioned to seeds than N derived from soil.

Description: Background and aims Global nitrogen (N) deposition can affect root morphology, anatomy and thus the water uptake by trees. The variation in uptake with the induced changes in root morphology and anatomy, however, is not clear. Methods We grew seedlings of Pinus tabuliformis for two years across an N gradient (0–12 g N m −2 y −1 ) and analysed the associated changes between the root hydraulic conductance (RHC) and the morphology and anatomy of different root orders. Results (1) The RHC increased with N addition and reached the peak at 9 g N m −2 y −1 . (2) The number, length, biomass and mycorrhizal colonization rates of the fine roots (first two order roots) decreased with N addition. (3) The stele diameter and tracheid diameter of the coarse roots (third and higher root orders) increased with N addition. (4) Decreasing root quantity and increasing physiological activity (i.e. RHC and respiration rates) induced by N addition may be a trade-off between root morphology and physiology plastic. Conclusions The morphological, anatomical and physiological responses of the different root orders to N addition increased the water uptake, transport, and whole-seedling growth. These heterogeneous responses should be incorporated into models of root uptake and growth in forest ecosystems.

Description: Aim A-nine month of field trail was conducted to investigate the accumulation capacity of cadmium and zinc by Arabidopsis halleri spp. germmifera . Methods An experimental site moderately contaminated with Zn and Cd was chosen to evaluate the field traits of seed sowing and seedling transplantation. The exchangeable fraction and total Cd and Zn in the soil and the concentration in harvested plants were determined by inductively coupled plasma mass spectrometer. Results The shoot biomass of A. halleri ssp. germmifera increased after more than 8 months of cultivation, and it was approximately 2.13 t ha −1 in the seedling transplantation group, which was more than three times higher than in the seed sowing group. The lower ratio of Zn/Cd concentration in plants than in the soil and the higher bioaccumulation factor indicated that A. halleri ssp. germmifera has a higher uptake efficiency for Cd than for Zn. In total, A. halleri ssp. germmifera removed 18.20 kg Cd ha −1 and 27.38 kg Zn ha −1 from the soil. After nine months of growth, A. halleri ssp. germmifera extracted (22.87 ± 9.21) % of total Cd concentration and (2.99 ± 0.94) % of total Zn concentration from the soil. However, no significant decrease of exchangeable concentration was shown in soil Cd and Zn, which revealed that the uptake of Cd and Zn by A. halleri ssp. germmifera not only came from exchangeable forms but also from non-exchangeable fractions. Conclusions The results of present study indicated that A. halleri ssp. germmifera can be used to efficiently remove different forms of Cd from contaminated land.

Description: Background and aims The changes in the characteristics of Panicum virgatum , an exotic invasive species, after invading various plant communities on the Loess Plateau in China and the main soil nutrient factors in these communities closely associated with invasion remain unclear. Methods A pot culture experiment was carried out to simulate the changes in photosynthesis, biomass, and biomass allocation in P. virgatum and to identify the main soil nutrient factors in various soils collected from local plant communities. P. virgatum was grown in soils collected from communities of P. virgatum (PS treatment), Setaria viridis (SS treatment), Bothriochloa ischaemum (BS treatment), and Artemisia sacrorum (AS treatment) and in a mixed soil from the communities of S. viridis , B. ischaemum , and A. sacrorum (MS treatment). Results Photosynthesis in P. virgatum differed significantly among the soil treatments. Net photosynthetic rate, stomatal conductance, and photochemical efficiency (Fv/Fm) were highest in PS, whereas single-photon avalanche diode values were highest in PS and SS. The variation of biomass differed significantly in different tissues of P. virgatum in the treatments. Leaf and stem biomasses were highest in PS and SS, and root biomass was highest in PS and MS. Total biomass differed significantly among the treatments, except between BS and MS. Both the leaf to total and stem to total biomass ratios were highest in AS and SS, but the root to total biomass ratio was lowest in these two treatments. A constrained redundancy analysis and a path analysis suggested that the water-soluble nitrate-nitrogen (W-NN) concentration of the soil could significantly affect photosynthesis, biomass, and biomass allocation in P. virgatum . Conclusions Photosynthesis, biomass, and biomass allocation in P. virgatum differed significantly when grown in soils from different local plant communities on the Loess Plateau. The soil W-NN concentration in these local plant communities likely has a large impact on the invasive success of P. virgatum .

Description: Background and aims Changes in net primary productivity in response to climate change are likely to affect litter inputs to forest soil. However, feedbacks between changes in litter input and soil carbon dynamics remain poorly understood in tropical and subtropical forests. This study aims to test whether the effects of litter manipulation on soil respiration differ between natural and plantation forests. Methods Soil respiration, soil properties, fine root biomass and enzyme activity were measured in adjacent plots with doubling vs. eliminating litter input in both natural and plantation forests of Castanopsis carlesii in southern China. Results After only 3 years of litter manipulation, the magnitude of change in soil respiration was greater in response to a doubling of the litter input (+24%) than to the elimination of litter input (−15%) in the natural forest, possibly due to a positive priming effect on decomposition of soil organic carbon (SOC). The quick and intense priming effect was corroborated by elevated enzyme activities for five of the six enzymes analyzed. In contrast, the response to litter removal (−31%) was greater than the response to litter addition (1%; not significant) in the plantation forest. The lack of positive priming in the plantation forest may be related to its lower soil fertility, which could not meet the demand of soil microbes, and to its high clay content, which protected SOC from microbial attack. The positive priming effect in the natural forest but not plantation forest of C. carlesii is also consistent with the significant declines in total soil carbon observed following litter addition in the natural forest but not the plantation forest. Conclusions Increases in aboveground litter production may trigger priming effects and subsequently transfer more soil carbon to atmospheric CO 2 in the natural forest but not in the plantation forest with low fertility. Changes in litter inputs resulting from global change drivers may have different impacts on natural and plantation forests.

Description: Background and aims 15 Nitrogen balances are useful tools to increase our understanding of soil N processes and the efficiency of plants in taking up animal urine-N. This study aimed to investigate the effect of forage type on the fate of autumn-applied urine-N (700 kg N ha −1 ). Methods In a 17-month lysimeter study the recoveries of 15 N in herbage, leachate, nitrous oxide, and soil + roots (at the end of the trial) were determined for three forage types: perennial ryegrass-white clover (RGWC, Lolium perenne L. -Trifolium repens L.), Italian ryegrass (Italian RG, Lolium multiflorum Lam.), and lucerne ( Medicago sativa L.) under simulated autumn grazing. Results We recovered 81.5–85.5% of the 15 N applied in the urine. Italian RG had the greatest amount of urine- 15 N recovered in the herbage (49.5%) and the least recovered in leachate (16.8%) making it the most efficient of the three forages for utilizing urine-N. For RGWC, herbage and leachate recoveries were 40.1% and 23.7% , respectively. Lucerne was the least efficient with recoveries of 15 N in herbage and leachate equaling 18.4% and 52.5%, respectively. Conclusions This study demonstrates that Italian ryegrass could offer benefits in reducing urine-N leaching losses through its ability to take up more urine-N over winter.

Description: Aims Successful inoculation of legume crops with rhizobia depends on dominating nodule occupancy with highly efficient strains. The aim of this study was to develop a rapid and reliable conventional PCR methodology to specifically detect an elite Bradyrhizobium strain in root nodule extracts from soil-grown cowpea plants. Methods The draft genome sequence of Bradyrhizobium pachyrhizi BR 3262 was compared to the closely related strain PAC 48 T . BR 3262-specific regions were selected to design specific primer pairs, which were tested with respect to PCR amplification specificity and efficiency on extracted DNA, bacterial cells and root nodules from cowpea plants grown under gnotobiotic conditions and in soil. Results Eleven designed primer pairs were specific for BR 3262 amplification and two of them (pairs 2645 and 2736) were highly sensitive and selected for further analyses. Experiments with gnotobiotic and soil-grown plants showed that both primer pairs were suitable to reliably determine nodule occupancy and confirmed the competitiveness of strain BR 3262 in natural soil. Conclusions Primer pairs 2645 and 2736 are novel tools to accompany the fate of strain BR 3262 in inoculation experiments of cowpea in soil. This strategy should be applicable to other rhizobium/legume symbioses in the field.

Description: Background and aims The intrinsic nitrogen (N) supply capacity of soil is central to understanding the productivity of natural plant communities, and essential in the context of determining optimal fertilization rates for agricultural soils. However, it is largely unknown how nutrient availability affects plant mediated priming effects driving soil organic matter mineralisation and associated N-fluxes. Methods We applied continuous, steady-state 13 C–CO 2 labelling to Lolium perenne grown in high and low productivity grassland soils to allow quantification of SOM- and root-derived soil CO 2 efflux. Nutrient treatments (N, P and K) were applied as repeated additions to soils, and impacts on source partitioned soil CO 2 efflux were assessed relative to unamended planted and fallow soils. Plants were clipped to uniform height at weekly intervals. Results Increasing nutrient availability in both soils resulted in a reduction in plant-mediated SOM mineralisation and clipping of plants greatly lowered root-derived respiration but increased SOM mineralisation. Nutrient addition to fallow systems had no effect on SOM mineralisation in either soil. Plant growth stimulated SOM priming, concurrent mobilisation of N from SOM and subsequent plant N uptake in the high productivity soil. Priming was not observed in the low productivity soil due to its greater inherent organic matter stability, resulting in lowered plant-mediated and basal SOM mineralisation. Conclusions That addition of nutrients reduced SOM mineralisation in planted systems but had no effect in fallow systems is indicative of nutrient availability specifically altering plant-mediated priming of SOM mineralisation. We suggest that plant-soil interactions mediating priming effects are an important determinant of productivity and that the magnitudes of these effects are modified by nutrient availability and soil-specific controls.

Description: Aims Our aim was to examine how soil type and precipitation affect fine-root abundance in savanna ecosystems across Kruger National Park (KNP), South Africa. Methods Fine-root distributions were measured in four sites that represent the natural factorial combination of soil types (basalt-derived clay or granite-derived sand) and precipitation regimes [wet (~750 mm mean annual precipitation) or dry (~500 mm mean annual precipitation)] that occur in KNP. Root area and biomass (at soil depths of 0–75 cm) were estimated from measurements of root number, length and width in images from minirhizotron tubes at each site. Measurements were made during one mid-season sampling during three subsequent years. Results Fine-root area was more than twice as large in clay (2.3 ± 0.0 mm 2  cm −2 ) than sand (0.8 ± 0.3 mm 2  cm −2 ) sites but did not differ between wet and dry sites. Root number, length and width, used to derive area, showed similar patterns to fine-root area. Fine-root biomass estimated from these values was 5.5 ± 0.6 Mg ha −1 in clay sites and 2.2 ± 0.9 Mg ha −1 in sand sites. Conclusions Across the four sites, a change from sand to clay soils had a greater effect on fine-root abundance and distributions than a 50% increase in precipitation from dry to wet sites. Results highlight the importance of soil properties on root dynamics and carbon pools in the region.

Description: Aims Drought is the major constraint to rainfed rice productivity in South Asia, but few reports provide detailed characterization of the soil properties related to drought stress severity in the region. The aim of the study was to provide a compilation of drought breeding network sites and their respective levels of drought stress, and to relate soil parameters with yield reduction by drought. Methods This study characterized levels of drought stress and soil nutrient and physical properties at 18 geographically distributed research station sites involved in rice varietal screening in Bangladesh, India, and Nepal, as well as at farmers’ fields located near the research stations. Results Based on soil resistance to penetration profiles, a hardpan was surprisingly absent at about half of the sites characterized. Significant relationships of depth of compaction and yield reduction by drought indicated the effects of soil puddling on susceptibility to cracking, rather than water retention by hardpans, on plant water availability in this region. The main difference between research stations and nearby farmers’ fields was in terms of soil compaction. Conclusions These results present an initiative for understanding the range of severities of reproductive-stage drought stress in drought-prone rainfed lowland rice-growing areas in South Asia.