111 research outputs found
Assessing the influence of rice roots and root exudates on nitrogen mineralization using a novel protocol
Classical nitrogen (N) mineralization experiments are done using uncropped soil, thus completely neglecting the influence of roots and root exudates. Therefore, experiments were conducted at two field sites in Bangladesh during ‘boro’ season (winter rice), using two rice cultivars (BRRI Dhan 29 and BINA Dhan6) to investigate the influence of rice roots and root exudates on N mineralization. Rice cultivars were transplanted in three replicated plots maintaining 25 x 15cm spacing along with three replicated uncropped plots as control. A novel method was used to identify the most suitable location to assess N mineralization in soil having actively growing rice plants. For this purpose, soil samples were collected from three locations in soil namely; 1) rhizosphere (0cm, at the rhizosphere), 2) middle of the two plants (7.5cm apart from rhizosphere) and 3) middle of two rows (12.5cm apart from rhizosphere). There was significant stimulatory effect of rice roots and root exudates on N mineralization at both filed sites. Significant influences of rice varieties were also observed, with BINA Dhan 6 having greater influence on N mineralization than BRRI Dhan 29. Sampling location also had a significant effect on measured N mineralization. The highest stimulatory effects of rice roots and root exudates were recorded when soil was sampled from rhizosphere. Sampling between the plants and between the rows had similar effects. In conclusion, rice roots and root exudates had a large influence on N mineralization and the best sampling location to determine the effects of actively growing rice roots and root exudates on N mineralization, was the rice rhizosphere
Tropical Andosol organic carbon quality and degradability in relation to soil geochemistry as affected by land use
Land use is recognized to impact soil geochemistry on the centennial to millennial timescale, with implications for the distribution and stability
of soil organic carbon (SOC). Young volcanic soils in tropical areas are subject to much faster pedogenesis, noticeable already on the centennial or
even decadal timescale. As land use is a recognized factor for soil formation, it is thus conceivable that even relatively recent land use conversion in
such areas would already bear a significant impact on the resulting formed soils., e.g., in terms of content of pedogenic oxides. Very scarce
observational evidence exists, so such indirect implications of land use on SOC cycling are largely unknown. We here investigated SOC fractions,
substrate-specific mineralization (SOC or added plant residue), and net priming of SOC as a function of forest or agricultural land use on Indonesian
volcanic soils. The content of oxalate-extracted Al (Alo) correlated well with organic carbon (OC) associated with sand-sized aggregates,
particularly in the subsoil. The proportion of SOC in sand-sized ultrasonication-resistant (400 J mL−1) aggregates was also higher in
agricultural land use compared to pine forest land use, and a likewise contrast existed for Alo. These combined observations suggest that enhanced
formation of Al (hydr)oxides promoted aggregation and physical occlusion of OC. This was, importantly, also consistent with a relatively lesser
degradability of SOC in the agricultural sites, though we found no likewise difference in degradability of added 13C-labeled ryegrass or
in native SOC priming between the pine forest and agricultural land uses. We expected that amorphous Al content under agricultural land use
would mainly have promoted mineral association of SOC compared to under pine forest land use but found no indications for this. Improved small-scale
aggregation of tropical Andosols caused by conversion to agriculture and high carbon input via organic fertilizer may thus partially counter the
otherwise expectable decline of SOC stocks following cultivation. Such indirect land use effects on the SOC balance appeared relevant for correct
interpretation and prediction of the long-term C balance of (agro)ecosystems with soil subject to intense development, like the here-studied tropical Andosols.</p
Early season N<sub>2</sub>O emissions under variable water management in rice systems: source-partitioning emissions using isotope ratios along a depth profile
Soil moisture strongly affects the balance between nitrification, denitrification
and N2O reduction and therefore the nitrogen (N) efficiency and N
losses in agricultural systems. In rice systems, there is a need to improve
alternative water management practices, which are designed to save water and
reduce methane emissions but may increase N2O and decrease nitrogen
use efficiency. In a field experiment with three water management treatments,
we measured N2O
isotope ratios of emitted and pore air N2O
(δ15N, δ18O and site preference, SP) over the
course of 6 weeks in the early rice growing season. Isotope ratio
measurements were coupled with simultaneous measurements of pore water
NO3-, NH4+, dissolved organic carbon (DOC), water-filled pore space (WFPS) and soil redox potential (Eh) at three soil depths.
We then used the relationship between SP × δ18O-N2O and
SP × δ15N-N2O in simple two end-member
mixing models to evaluate the contribution of nitrification, denitrification
and fungal denitrification to total N2O emissions and to estimate
N2O reduction rates. N2O emissions were higher in a
dry-seeded + alternate wetting and drying (DS-AWD) treatment relative to
water-seeded + alternate wetting and drying (WS-AWD) and
water-seeded + conventional flooding (WS-FLD) treatments. In the DS-AWD
treatment the highest emissions were associated with a high contribution from
denitrification and a decrease in N2O reduction, while in the WS
treatments, the highest emissions occurred when contributions from
denitrification/nitrifier denitrification and nitrification/fungal
denitrification were more equal. Modeled denitrification rates appeared to be
tightly linked to nitrification and NO3- availability in all
treatments; thus, water management affected the rate of denitrification and
N2O reduction by controlling the substrate availability for each
process (NO3- and N2O), likely through changes in
mineralization and nitrification rates. Our model estimates of mean
N2O reduction rates match well those observed in 15N
fertilizer labeling studies in rice systems and show promise for the use of
dual isotope ratio mixing models to estimate N2 losses.</p
Monitoring and Scoring Counter-Diffusion Protein Crystallization Experiments in Capillaries by in situ Dynamic Light Scattering
In this paper, we demonstrate the feasibility of using in situ Dynamic Light Scattering (DLS) to monitor counter-diffusion crystallization experiments in capillaries. Firstly, we have validated the quality of the DLS signal in thin capillaries, which is comparable to that obtained in standard quartz cuvettes. Then, we have carried out DLS measurements of a counter-diffusion crystallization experiment of glucose isomerase in capillaries of different diameters (0.1, 0.2 and 0.3 mm) in order to follow the temporal evolution of protein supersaturation. Finally, we have compared DLS data with optical recordings of the progression of the crystallization front and with a simulation model of counter-diffusion in 1D
Thresholds for adding degraded tropical forest to the conservation estate
Logged and disturbed forests are often viewed as degraded and depauperate environments compared with primary forest. However, they are dynamic ecosystems1 that provide refugia for large amounts of biodiversity2,3, so we cannot afford to underestimate their conservation value4. Here we present empirically defined thresholds for categorizing the conservation value of logged forests, using one of the most comprehensive assessments of taxon responses to habitat degradation in any tropical forest environment. We analysed the impact of logging intensity on the individual occurrence patterns of 1,681 taxa belonging to 86 taxonomic orders and 126 functional groups in Sabah, Malaysia. Our results demonstrate the existence of two conservation-relevant thresholds. First, lightly logged forests (68%) of their biomass removed, and these are likely to require more expensive measures to recover their biodiversity value. Overall, our data confirm that primary forests are irreplaceable5, but they also reinforce the message that logged forests retain considerable conservation value that should not be overlooked
Thresholds for adding degraded tropical forest to the conservation estate
Logged and disturbed forests are often viewed as degraded and depauperate environments compared with primary forest. However, they are dynamic ecosystems1 that provide refugia for large amounts of biodiversity2,3, so we cannot afford to underestimate their conservation value4. Here we present empirically defined thresholds for categorizing the conservation value of logged forests, using one of the most comprehensive assessments of taxon responses to habitat degradation in any tropical forest environment. We analysed the impact of logging intensity on the individual occurrence patterns of 1,681 taxa belonging to 86 taxonomic orders and 126 functional groups in Sabah, Malaysia. Our results demonstrate the existence of two conservation-relevant thresholds. First, lightly logged forests (68%) of their biomass removed, and these are likely to require more expensive measures to recover their biodiversity value. Overall, our data confirm that primary forests are irreplaceable5, but they also reinforce the message that logged forests retain considerable conservation value that should not be overlooked
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