Microbial response to rhizodeposition depending on water regimes in paddy soils

Rhizodeposit-carbon (rhizo-C) serves as a primary energy and C source for microorganisms in the rhizosphere. Despite important progress in understanding the fate of rhizo-C in upland soils, little is known about microbial community dynamics associated with rhizo-C in flooded soils, especially depending on water regimes in rice systems. In this study, rice grown under non-flooded, continuously flooded and alternating water regimes was pulse labeled with ¹³CO₂ and the incorporation of rhizo-C into specific microbial groups was determined by ¹³C in phospholipid fatty acids (PLFAs) at day 2 and 14 after the labeling.A decreased C released from roots under continuously flooded condition was accompanied with lower total ¹³C incorporation into microorganisms compared to the non-flooded and alternating water regimes treatments. Continuous flooding caused a relative increase of ¹³C incorporation in Gram positive bacteria (i14:0, i15:0, a15:0, i16:0, i17:0, a17:0). In contrast, Gram negative bacteria (16:1ω7c, 18:1ω7c, cy17:0, cy 19:0) and fungi (18:2ω6, 9c, 18:1ω9c) showed greater rhizo-C incorporation coupled with a higher turnover under non-flooded and alternating water regimes treatments. These observations suggest that microbial groups processing rhizo-C differed among rice systems with varying water regimes. In contrast to non-flooded and alternating water regimes, there was little to no temporal ¹³C change in most microbial groups under continuous flooding condition between day 2 and 14 after the labeling, which may demonstrate slower microbial processing turnover. In summary, our findings indicate that belowground C input by rhizodeposition and its biological cycling was significantly influenced by water regimes in rice systems.     

Rhizodeposition: Its contribution to microbial growth and carbon and nitrogen turnover within the rhizosphere

A greenhouse rhizobox experiment was carried out to investigate the fate and turnover of ¹³C‐ and ¹⁵N‐labeled rhizodeposits within a rhizosphere gradient from 0–8 mm distance to the roots of wheat. Rhizosphere soil layers from 0–1, 1–2, 2–3, 3–4, 4–6, and 6–8 mm distance to separated roots were investigated in an incubation experiment (42 d, 15°C) for changes in total C and N and that derived from rhizodeposition in total soil, in soil microbial biomass, and in the 0.05 M K₂SO₄–extractable soil fraction. CO₂‐C respiration in total and that derived from rhizodeposition were measured from the incubated rhizosphere soil samples. Rhizodeposition C was detected in rhizosphere soil up to 4–6 mm distance from the separated roots. Rhizodeposition N was only detected in the rhizosphere soils up to 3–4 mm distance from the roots. Microbial biomass C and N was increased with increasing proximity to the separated roots. Beside ¹³C and ¹⁵N derived from rhizodeposits, unlabeled soil C and N (native SOM) were incorporated into the growing microbial biomass towards the roots, indicating a distinct acceleration of soil organic matter (SOM) decomposition and N immobilization into the growing microbial biomass, even under the competition of plant growth. During the soil incubation, microbial biomass C and N decreased in all samples. Any decrease in microbial biomass C and N in the incubated rhizosphere soil layers is attributed mainly to a decrease of unlabeled (native) C and N, whereas the main portion of previously incorporated rhizodeposition C and N during the plant growth period remained immobilized in the microbial biomass during the incubation. Mineralization of native SOM C and N was enhanced within the entire investigated rhizosphere gradient. The results indicate complex interactions between substrate input derived from rhizodeposition, microbial growth, and accelerated C and N turnover, including the decomposition of native SOM (i.e., rhizosphere priming effects) at a high spatial resolution from the roots.     

Rhizodeposition-induced decomposition increases N availability to wild and cultivated wheat genotypes under elevated CO2

Elevated CO₂ may increase nutrient availability in the rhizosphere by stimulating N release from recalcitrant soil organic matter (SOM) pools through enhanced rhizodeposition. We aimed to elucidate how CO₂-induced increases in rhizodeposition affect N release from recalcitrant SOM, and how wild versus cultivated genotypes of wheat mediated differential responses in soil N cycling under elevated CO₂. To quantify root-derived soil carbon (C) input and release of N from stable SOM pools, plants were grown for 1 month in microcosms, exposed to ¹³C labeling at ambient (392 μmol mol⁻¹) and elevated (792 μmol mol⁻¹) CO₂ concentrations, in soil containing ¹⁵N predominantly incorporated into recalcitrant SOM pools. Decomposition of stable soil C increased by 43%, root-derived soil C increased by 59%, and microbial-¹³C was enhanced by 50% under elevated compared to ambient CO₂. Concurrently, plant ¹⁵N uptake increased (+7%) under elevated CO₂ while ¹⁵N contents in the microbial biomass and mineral N pool decreased. Wild genotypes allocated more C to their roots, while cultivated genotypes allocated more C to their shoots under ambient and elevated CO₂. This led to increased stable C decomposition, but not to increased N acquisition for the wild genotypes. Data suggest that increased rhizodeposition under elevated CO₂ can stimulate mineralization of N from recalcitrant SOM pools and that contrasting C allocation patterns cannot fully explain plant mediated differential responses in soil N cycling to elevated CO₂.     

Fate of C- and N-labelled rhizodeposition of Lolium perenne as function of the distance to the root surface

A greenhouse rhizobox experiment was carried out to quantify the incorporation of ¹³C- and ¹⁵N-labelled rhizodeposits into different soil pools, especially into the rhizosphere microbial biomass, with increasing distances to the root surface of Lolium perenne. Five layers were analysed over 0-4.2 mm distance to an artificial root surface. C and N derived from rhizodeposition were 4.2% of total C and 2.8% of total N in soil at 0-1.0 mm distance and decreased rapidly with increasing distance. Microbial biomass C and N increased significantly towards the roots. At 0-1.0 mm distance microbial biomass C and N accounted for 66% and 29% of C and N derived from rhizodeposition, respectively. These percentages declined with increasing distance to the roots, but were still traceable up to 4.2 mm distance. Only small amounts of root released C and N were found in the 0.05 M K₂SO₄-extractable fraction. Extractable C and N derived from rhizodeposition varied around means of 4% of total C and N derived from rhizodeposition and increased only marginally with increasing distance to the roots. C derived from rhizodeposition in the non-extractable soil organic matter increased from 65 to 89% of total C derived from rhizodeposition at 0-3.4 mm distance. Conversely, microbial biomass C derived from rhizodeposition decreased from 33 to 4%. N derived from rhizodeposition in the non-extractable soil organic matter increased from 61 to 79% of total N derived from rhizodeposition at 0-2.6 mm distance, followed by a decline to roughly 55% in the two outer layers. Microbial biomass N decreased from 37 to 16% at 0-2.6 mm distance, followed by an increase to roughly 41% in the two outer layers. The C/N ratio of total C and N derived from rhizodeposition as well as that of extractable C and N derived from rhizodeposition increased with increasing distance to the roots to values above 30. In contrast, the C/N ratio of incorporated rhizodeposition C and N into the microbial biomass decreased to values less than 5 at 2.6-4.2 mm distance. The data indicate differential microbial response to C and N derived from rhizodeposition at a high spatial resolution from the root surface. The turnover of C and N derived from rhizodeposition in the rhizosphere as a function of the distance to the root surface is discussed.     

Release of C and N from roots of peas and oats and their availability to soil microorganisms

Nutrient mobilisation in the rhizosphere is driven by soil microorganisms and controlled by the release of available C compounds from roots. It is not known how the quality of release influences this process in situ. Therefore, the present study was conducted to investigate the amount and turnover of rhizodeposition, in this study defined as root-derived C or N present in the soil after removal of roots and root fragments, released at different growth stages of peas (Pisum sativum L.) and oats (Avena sativa L.). Plants were grown in soil columns placed in a raised bed under outdoor conditions and simultaneously pulse labelled in situ with a ¹³C-glucose-¹⁵N-urea solution using a stem feeding method. After harvest, ¹³C and ¹⁵N was recovered in plant parts and soil pools, including the microbial biomass. Net rhizodeposition of C and N as a percentage of total plant C and N was higher in peas than in oats. Moreover, the C-to-N ratio of the rhizodeposits was lower in peas, and a higher proportion of the microbial biomass and inorganic N was derived from rhizodeposition. These results suggest a positive plant-soil feedback shaping nutrient mobilisation. This process is driven by the C and N supply of roots, which has a higher availability in peas than in oats.     

Distribution and turnover of recently fixed photosynthate in ryegrass rhizospheres

The cycling of root-deposited photosynthate (rhizodeposition) through the soil microbial biomass can have profound influences on plant nutrient availability. Currently, our understanding of microbial dynamics associated with rhizosphere carbon (C) flow is limited. We used a ¹³C pulse-chase labeling procedure to examine the flow of photosynthetically fixed ¹³C into the microbial biomass of the bulk and rhizosphere soils of greenhouse-grown annual ryegrass (Lolium multiflorum Lam.). To assess the temporal dynamics of rhizosphere C flow through the microbial biomass, plants were labeled either during the transition between active root growth and rapid shoot growth (Labeling Period 1), or nine days later during the rapid shoot growth stage (Labeling Period 2). Although the distribution of ¹³C in the plant/soil system was similar between the two labeling periods, microbial cycling of rhizodeposition differed between labeling periods. Within 24 h of labeling, more than 10% of the ¹³C retained in the plant/soil system resided in the soil, most of which had already been incorporated into the microbial biomass. From day 1 to day 8, the proportion of ¹³C in soil as microbial biomass declined from about 90 to 35% in rhizosphere soil and from about 80 to 30% in bulk soil. Turnover of ¹³C through the microbial biomass was faster in rhizosphere soil than in bulk soil, and faster in Labeling Period 1 than Labeling Period 2. Our results demonstrate the effectiveness of using ¹³C labeling to examine microbial dynamics and fate of C associated with cycling of rhizodeposition from plants at different phenological stages of growth.     

Carbon cycling in rice field ecosystems in the context of input, decomposition and translocation of organic materials and the fates of their end products (CO2 and CH4)

Rice fields are intensively managed, unique agroecosystems, where soil flooding is general performance for rice cultivation. Flooding the field results in reductive soil conditions, under which decomposition of organic materials proceeds during the period of rice cultivation. A large variety of organic materials are incorporated into rice soils according to field management. In this review, the kind and abundance of organic materials entering carbon cycling in the rice field ecosystem are evaluated first. Then, decomposition of plant residues and soil organic matter in rice fields is reviewed quantitatively. Decomposition of plant residues is shown to be the active process in carbon cycling in rice fields. Rice releases photosynthates into the rhizosphere (rhizodeposition), and they follow a different avenue of decomposition in soil from that of plant residues. Incorporation of rhizodeposition into microbial biomass and soil organic matter during the period of rice cultivation, and their fates after harvesting are evaluated quantitatively from ¹³C pulse labeled experiments. Percolating water transports inorganic and organic carbon from the plow layer to the subsoil layer. The amounts of their transport and accumulation in the subsoil layer are evaluated in relation to the amounts of soil organic C in the plow layer. Not only CO₂ but also CH₄ are produced in the decomposition process of organic materials in flooded rice fields. CH₄ evolution from rice fields is of global concern from the viewpoint of global warming. Origins of CH₄ evolved from rice fields are estimated first, followed by the fates of CH₄ in rice field ecosystems. Rhizodeposition is shown to be the main origin of CH₄ evolved from rice fields. Evolution to the atmosphere is not the sole pathway of CH₄ produced in rice fields. The amounts of CH₄ retained in soil, percolated to the subsoil layer and decomposed in soil are evaluated in the context of the amounts of CH₄ efflux. Thus, this review focuses on carbon cycling in the rice field ecosystem from the viewpoints of input, decomposition, and translocation of organic materials and the fates of their end products (CO₂ and CH₄).     

Effects of nitrogen fertilization on pot‐grown wheat photosynthate partitioning within intensively farmed soil determined by 13C pulse‐labeling

Understanding rhizodeposited carbon (C) dynamics of winter wheat (Triticum aestivum L.) is important for improving soil fertility and increasing soil C stocks. However, the effects of nitrogen (N) fertilization on photosynthate C allocation to rhizodeposition of wheat grown in an intensively farmed alkaline soil remain elusive. In this study, pot‐grown winter wheat under N fertilization of 250 kg N ha^?1 was pulse‐labeled with ¹³CO₂ at tillering, elongation, anthesis, and grain‐filling stages. The ¹³C in shoots, roots, soil organic carbon (SOC), and rhizosphere‐respired CO₂ was measured 28 d after each ¹³C labeling. The proportion of net‐photosynthesized ¹³C recovered (shoots + roots + soil + soil respired CO₂) in the shoots increased from 58–64% at the tillering to 86–91% at the grain‐filling stage. Likewise, the proportion in the roots decreased from 21–28% to 2–3%, and that in the SOC pool increased from 1–2% to 6–7%. However, the ¹³C respired CO₂ allocated to soil peaked (17–18%) at the elongation stage and decreased to 6–8% at the grain‐filling stage. Over the entire growth season of wheat, N fertilization decreased the proportion of net photosynthate C translocated to the below‐ground pool by about 20%, but increased the total amount of fixed photosynthate C, and therefore increased the below‐ground photosynthate C input. We found that the chase period of about 4 weeks is sufficient to accurately monitor the recovery of ¹³C after pulse labeling in a wheat–soil system. We conclude that N fertilization increased the deposition of photoassimilate C into SOC pools over the entire growth season of wheat compared to the control treatment.     

Rice rhizodeposition and its utilization by microbial groups depends on N fertilization

Rhizodeposits have received considerable attention, as they play an important role in the regulation of soil carbon (C) sequestration and global C cycling and represent an important C and energy source for soil microorganisms. However, the utilization of rhizodeposits by microbial groups, their role in the turnover of soil organic matter (SOM) pools in rice paddies, and the effects of nitrogen (N) fertilization on rhizodeposition are nearly unknown. Rice (Oryza sativa L.) plants were grown in soil at five N fertilization rates (0, 10, 20, 40, or 60 mg N kg^?1 soil) and continuously labeled in a ¹³CO₂ atmosphere for 18 days during tillering. The utilization of root-derived C by microbial groups was assessed by ¹³C incorporation into phospholipid fatty acids. Rice shoot and root biomass strongly increased with N fertilization. Rhizodeposition increased with N fertilization, whereas the total ¹³C incorporation into microorganisms, as indicated by the percentage of ¹³C recovered in microbial biomass, decreased. The contribution of root-derived ¹³C to SOM formation increased with root biomass. The ratio of ¹³C in soil pools (SOM and microbial biomass) to ¹³C in roots decreased with N fertilization showing less incorporation and faster turnover with N. The ¹³C incorporation into fungi (18:2ω6,9c and 18:1ω9c), arbuscular mycorrhizal fungi (16:1ω5c), and actinomycetes (10Me 16:0 and 10Me 18:0) increased with N fertilization, whereas the ¹³C incorporation into gram-positive (i14:0, i15:0, a15:0, i16:0, i17:0, and a17:0) and gram-negative (16:1ω7c, 18:1ω7c, cy17:0, and cy19:0) bacteria decreased with N fertilization. Thus, the uptake and microbial processing of root-derived C was affected by N availability in soil. Compared with the unfertilized soil, the contribution of rhizodeposits to SOM and microorganisms increased at low to intermediate N fertilization rates but decreased at the maximum N input. We conclude that belowground C allocation and rhizodeposition by rice, microbial utilization of rhizodeposited C, and its stabilization within SOM pools are strongly affected by N availability: N fertilization adequate to the plant demand increases C incorporation in all these polls, but excessive N fertilization has negative effects not only on environmental pollution but also on C sequestration in soil.     

Incorporation of 13C-labelled rice rhizodeposition carbon into soil microbial communities under different water status

The overall processes by which carbon is fixed by plants in photosynthesis then released into the soil by rhizodeposition and subsequently utilized by soil micro-organisms, links the atmospheric and soil carbon pools. The objective of this study was to determine the plant derived ¹³C incorporated into the phospholipid fatty acid (PLFA) pattern in paddy soil, to test whether utilization of rice rhizodeposition carbon by soil micro-organisms is affected by soil water status. This is essential to understand the importance of flooded conditions in regulating soil microbial community structure and activity in wetland rice systems. Rice plants were grown in soil derived from a paddy system under controlled irrigation (CI), or with continuous waterlogging (CW). Most of the ¹³C-labelled rice rhizodeposition carbon was distributed into the PLFAs 16:0, 18:1ω7 and 18:1ω9 in both the CW and CI treatments. The bacterial PLFAs i15:0 and a15:0, both indicative of gram positive bacteria, were relatively more abundant in the treatments without rice plants. When rice plants were present rates of ¹³C-incorporation into i15:0 and a15:0 was slow; the microbes containing these PLFAs may derive most of their carbon from more recalcitrant C (soil organic matter). PLFAs, 18:1ω7 and 16:1ω7c, indicative of gram negative bacteria showed a greater amount incorporation of labelled plant derived carbon in the CW treatment. In contrast, 18:2ω6,9 indicative of fungi and 18:1ω9 indicative of aerobes but also potentially fungi and plant roots had greater incorporation in the CI treatment. The greater root mass concomitant with lower incorporation of ¹³C into the total PLFA pool in the CW treatment suggests that the microbial communities in wetland rice soil are limited by factors other than substrate availability in flooded conditions. In this study differing soil microbial communities were established through manipulating the water status of paddy soils. Steady state ¹³C labelling enabled us to determine that the microbial community utilizing plant derived carbon was also affected by water status.     

Influences of irrigation,nitrogen and zeolite management on the physicochemical properties of rice

The addition of zeolite (Z) to soils is increasingly being recognised as a way to enhance agricultural production and decrease fertilisation requirements and, hence, environmental costs. Meanwhile, the alternate wetting and drying irrigation (AWD) has become widely applied to reduce the water requirements of rice cultivation. However, limited information is available on their impacts on rice’s physicochemical properties. This study investigated an integrated irrigation, nitrogen (N) and Z rice production system and assessed its effects on the milling, appearance, nutrition, taste and cooking qualities of the rice grain produced. Compared with conventional flooding irrigation (CF), AWD-grown rice had slightly decreased milling and appearance qualities. Addition of Z increased rice protein content and slightly decreased eating quality without affecting milling, appearance and cooking qualities. The highest yields achieved under AWD (9.8 t ha^?1) and CF (8.9 t ha^?1) were achieved using 105 kg N and 10 t Z ha^?1, and 105 kg N and 5 t Z ha^?1, respectively. Compared with the flooding untreated control (using 157.5 kg N ha^?1 and no Z), these two treatment regimens required 27.8% and 8.1% less water, 33.3% less N fertiliser and increased yields by 10.6% and 0.6%, respectively, without measurably affecting rice grain quality.     

Influence of water regime on growth,yield, and nitrogen uptake of rice

Abstract

Greenhouse and field experiments were conducted to study the effects of water regime on growth of rice. The greenhouse experiment investigated the effects of two water regimes‐continuous flooding and flooding with soil drying between crops for 2 to 3 weeks on the growth of rice during six cropping (for six week each) on seven soils varying widely in total N contents (0.07 to 0.35%). The results averaged for the 7 soils indicated that the drymatter production or N uptake of rice was not affected by the water regimes during the six croppings.

The field experiment conducted during the dry season for two consecutive years (1976 and 1977) on a near neutral clay soil studied the effects of three water regimes (continuous flooding alternate flooding and soil drying every 2 weeks, and continuous flooding with 2 weeks mid season soil drying after 6 weeks of transplanting) and three levels of fertilizer N (0, 100 and 200 kg N/ha as urea) on grain yield and N uptake of rice. The results confirmed the absence of any significant reduction in grain yield or N uptake as a result of any of the soil drying treatments during the growing season on the unfertilized plots carrying a rice crop. On the plots fertilized with 100 or 200 kg N/na, alternate flooding and drying resulted in a significant depression in both grain yield and N uptake. Soil analysis supported heavy losses of N in the fertilized plots that underwent alternate flooding and soil drying apparently by nitrification and denitrification reactions.

The results of this study suggest that alternate flooding and drying of soils in the presence of established rice plants itiay not cause a significant loss of nitrogen in unfertilized plots although in plots fertilized with high rates of N the losses may be large as indicated by the performance of rice crop.     

干湿交替灌溉改善稻田根际氧环境进而促进氮素转化和水稻氮素吸收

[目的]阐明不同水氮管理模式下水稻根际内外氧环境变化特征及其对土壤碳氮转化和水稻氮吸收利用的影响,以期从稻田"根际氧环境"调控角度揭示适宜水氮耦合促进水稻生长和提高氮素利用效率的内在机制.[方法]在长期定位试验基础上,采用根箱模拟培养以及Unisense微电极系统和15N同位素示踪相结合的研究方法,以常规粳稻日本晴和常...     

Nitrogen rhizodeposition of young wheat plants under elevated CO<Subscript>2</Subscript> and drought stress

The objective of this study was to determine the effect of drought stress and elevated CO₂ concentrations around the shoots on N rhizodeposition of young wheat plants. In a pot experiment, the plant N pool was labeled through ¹⁵NH₃ application to shoots at nontoxic NH₃ concentrations, and the impact of low water supply (40% field capacity), elevated CO₂ (720 μmol mol⁻¹ CO₂), and the combination of both factors on the ¹⁵N distribution was studied. Total ¹⁵N rhizodeposition ranged from 5 to 11% of the total ¹⁵N recovered in the plant/soil system. Elevated CO₂ concentration as well as drought stress increased the belowground transport of N and increased the relative portion of N rhizodeposition on total ¹⁵N in the plant/soil system. However, while the increased N rhizodeposition with elevated CO₂ was the result of increased total belowground N transport, drought stress additionally increased the portion of ¹⁵N found in rhizodeposition vs roots. Elevated CO₂ intensified the effect of drought stress. The percentage of water soluble ¹⁵N in the ¹⁵N rhizodeposition was very low under all treatments, and it was significantly decreased by the drought-stressed treatments.     

Long-term fertilizer management in NERICA cultivated upland affects on soil bio-chemical properties

ABSTRACT

This study aims to characterize soil chemical properties and microbial biomass, greenhouse gas production, and organic matter dynamics in upland rice field as affected by the long-term fertilizer managements in Uganda. Soil total C (TC) and N (TN) contents were in the relatively smaller range under different fertilizer treatments, even after 20 crop seasons. However, available phosphate contents showed positive correlation with average yield of upland rice. Incubation experiments were conducted under aerobic or under flooding conditions to measure CO₂, methane, and nitrous oxide productions. After the incubation, soil samples were extracted to quantify nitrification rate for aerobic condition and ammonification rate for flooding condition. Soil microbial biomass carbon (MBC) and nitrogen were measured. Stable isotope ratio of ¹³C and ¹⁵N were also determined for the soil samples. CO₂ production potential under aerobic condition was higher than the flooding condition. The qCO₂ (CO₂/MBC) in the treatment applied with compost tended to be higher than the other treatments. Positive correlation between nitrous oxide production and nitrification was found. The delta ¹³C values of the soil samples indicated that the effect of C4 plants before rice cultivation still remained, while the contribution of biological N₂ fixation was little according to delta ¹⁵N values. These results indicate that soil microbial biomass in upland rice field of the long-term fertilizer experiment in Uganda was characterized with higher qCO₂. Greenhouse gas production was affected by fertilizer management, while soil organic C before the long-term experiment still remained in the experiment.     

水稻根际和周围土壤中微生物功能基因丰度与碳、氮状况及其通量之间的关系

Rapid nitrogen（N） transformations and losses occur in the rice rhizosphere through root uptake and microbial activities. However,the relationships between rice roots and rhizosphere microbes for N utilization are still unclear. We analyzed different N forms（NH＋4,NO-3, and dissolved organic N）, microbial biomass N and C, dissolved organic C, CH4 and N2O emissions, and abundance of microbial functional genes in both rhizosphere and bulk soils after 37-d rice growth in a greenhouse pot experiment. Results showed that the dissolved organic C was significantly higher in the rhizosphere soil than in the non-rhizosphere bulk soil, but microbial biomass C showed no significant difference. The concentrations of NH＋4, dissolved organic N, and microbial biomass N in the rhizosphere soil were significantly lower than those of the bulk soil, whereas NO-3in the rhizosphere soil was comparable to that in the bulk soil. The CH4 and N2O fluxes from the rhizosphere soil were much higher than those from the bulk soil. Real-time polymerase chain reaction analysis showed that the abundance of seven selected genes, bacterial and archaeal 16 S rRNA genes, amoA genes of ammonia-oxidizing archaea and ammonia-oxidizing bacteria, nosZ gene, mcrA gene, and pmoA gene, was lower in the rhizosphere soil than in the bulk soil, which is contrary to the results of previous studies. The lower concentration of N in the rhizosphere soil indicated that the competition for N in the rhizosphere soil was very strong, thus having a negative effect on the numbers of microbes. We concluded that when N was limiting, the growth of rhizosphere microorganisms depended on their competitive abilities with rice roots for N.     

Effect of Lime and Flooding on Phosphorus Availability and Rice Growth on Two Acidic Lowland Soils

Abstract

Loss of soil‐water saturation may impair growth of rainfed lowland rice by restricting nutrient uptake, including the uptake of added phosphorus (P). For acidic soils, reappearance of soluble aluminum (Al) following loss of soil‐water saturation may also restrict P uptake. The aim of this study was to determine whether liming, flooding, and P additions could ameliorate the effects of loss of soil‐water saturation on P uptake and growth of rice. In the first pot experiment, two acid lowland soils from Cambodia [Kandic Plinthaqult (black clay soil) and Plinthustalf (sandy soil)] were treated with P (45 mg P kg^?1 soil) either before or after flooding for 4 weeks to investigate the effect of flooding on effectiveness of P fertilizer for rice growth. After 4 weeks, soils were air dried and crushed and then wet to field capacity and upland rice was grown in them for an additional 6 weeks. Addition of P fertilizer before rather than after flooding depressed the growth of the subsequently planted upland rice. During flooding, there was an increase in both acetate‐extractable Fe and the phosphate sorption capacity of soils, and a close relationship between them (r²=0.96–0.98). When P was added before flooding, Olsen and Bray 1‐extractable P, shoot dry matter, and shoot P concentrations were depressed, indicating that flooding decreased availability of fertilizer P. A second pot experiment was conducted with three levels of lime as CaCO₃ [to establish pH (CaCl₂) in the oxidized soils at 4, 5, and 6] and four levels of P (0, 13, 26, and 52 mg P kg^?1 soil) added to the same two acid lowland rice soils under flooded and nonflooded conditions. Under continuously flooded conditions, pH increased to over 5.6 regardless of lime treatment, and there was no response of rice dry matter to liming after 6 weeks' growth, but the addition of P increased rice dry matter substantially in both soils. In nonflooded soils, when P was not applied, shoot dry matter was depressed by up to one‐half of that in plants grown under continuously flooded conditions. Under the nonflooded conditions, rice dry matter and leaf P increased with the addition of P, but less so than in flooded soils. Leaf P concentrations and shoot dry matter responded strongly to the addition of lime. The increase in shoot dry matter of rice with lime and P application in nonflooded soil was associated with a significant decline in soluble Al in the soil and an increase in plant P uptake. The current experiments show that the loss of soil‐water saturation may be associated with the inhibition of P absorption by excess soluble Al. By contrast, flooding decreased exchangeable Al to levels below the threshold for toxicity in rice. In addition, the decreased P availability with loss of soil‐water saturation may have been associated with a greater phosphate sorption capacity of the soils during flooding and after reoxidation due to occlusion of P within ferric oxyhydroxides formed.     

Dry-rewetting cycles regulate wheat carbon rhizodeposition,stabilization and nitrogen cycling

Drying and rewetting of soil can have large effects on carbon (C) and nitrogen (N) dynamics. Drying-rewetting effects have mostly been studied in the absence of plants, although it is well known that plant–microbe interactions can substantially alter soil C and N dynamics. We investigated for the first time how drying and rewetting affected rhizodeposition, its utilization by microbes, and its stabilization into soil (C associated with soil mineral phase). We also investigated how drying and rewetting influenced N mineralization and loss. We grew wheat (Triticum aestivum) in a controlled environment under constant moisture and under dry-rewetting cycles, and used a continuous ¹³C-labeling method to partition plant and soil organic matter (SOM) contribution to different soil pools. We applied a ¹⁵N label to the soil to determine N loss. We found that dry-rewetting decreased total input of plant C in microbial biomass (MB) and in the soil mineral phase, mainly due to a reduction of plant biomass. Plant derived C in MB and in the soil mineral phase were positively correlated (R² = 0.54; P = 0.0012). N loss was reduced with dry rewetting cycles, and mineralization increased after each rewetting event. Overall drying and rewetting reduced rhizodeposition and stabilization of new C, primary through biomass reduction. However, frequency of rewetting and intensity of drought may determine the fate of C in MB and consequently into the soil mineral phase. Frequency and intensity may also be crucial in stimulating N mineralization and reducing N loss in agricultural soils.     

Turnover of grain legume N rhizodeposits and effect of rhizodeposition on the turnover of crop residues

The turnover of N derived from rhizodeposition of faba bean (Vicia faba L.), pea (Pisum sativum L.) and white lupin (Lupinus albus L.) and the effects of the rhizodeposition on the subsequent C and N turnover of its crop residues were investigated in an incubation experiment (168 days, 15 °C). A sandy loam soil for the experiment was either stored at 6 °C or planted with the respective grain legume in pots. Legumes were in situ ¹⁵N stem labelled during growth and visible roots were removed at maturity. The remaining plant-derived N in soil was defined as N rhizodeposition. In the experiment the turnover of C and N was compared in soils with and without previous growth of three legumes and with and without incorporation of crop residues. After 168 days, 21% (lupin), 26% (faba bean) and 27% (pea) of rhizodeposition N was mineralised in the treatments without crop residues. A smaller amount of 15–17% was present as microbial biomass and between 30 and 55% of mineralised rhizodeposition N was present as microbial residue pool, which consists of microbial exoenzymes, mucous substances and dead microbial biomass. The effect of rhizodeposition on the C and N turnover of crop residues was inconsistent. Rhizodeposition increased the crop residue C mineralisation only in the lupin treatment; a similar pattern was found for microbial C, whereas the microbial N was increased by rhizodeposition in all treatments. The recovery of residual ¹⁵N in the microbial and mineral N pool was similar between the treatments containing only labelled crop residues and labelled crop residues + labelled rhizodeposits. This indicates a similar decomposability of both rhizodeposition N and crop residue N and may be attributable to an immobilisation of both N sources (rhizodeposits and crop residues) as microbial residues and a subsequent remineralisation mainly from this pool.Abbreviations C or N_dec C or N decomposed from residues - C or N_mic microbial C or N - C or N_micres microbial residue C or N - C or N_min mineralised C or N - C or N_input added C or N as crop residues and/or rhizodeposits - dfr derived from residues - dfR derived from rhizodeposition - Ndfr N derived from residues - NdfR N derived from rhizodeposition - N_loss losses of N derived from residues - SOM soil organic matter - WHC water holding capacity     

Microbial immobilisation of C rhizodeposits in rhizosphere and root-free soil under continuous C labelling of oats

A greenhouse experiment was conducted by growing oats (Avenasativa L.) in a continuously ¹³CO₂ labeled atmosphere. The allocation of ¹³C-labeled photosynthates in plants, microbial biomass in rhizosphere and root-free soil, pools of soil organic C, and CO₂ emissions were examined over the plant's life cycle. To isolate rhizosphere from root-free soil, plant seedlings were placed into bags made of nylon monofilament screen tissue (16 μm mesh) filled with soil. Two peaks of ¹³C in rhizosphere pools of microbial biomass and dissolved organic carbon (DOC), as well as in CO₂ emissions at the earing and ripeness stages were revealed. These ¹³C maxima corresponded to: (i) the end of rapid root growth and (ii) beginning of root decomposition, respectively. The δ¹³C values of microbial biomass were higher than those of DOC and of soil organic matter (SOM). The microbial biomass C accounted for up to 56 and 39% of ¹³C recovered in the rhizosphere and root-free soil, respectively. Between 4 and 28% of ¹³C assimilated was recovered in the root-free soil. Depending on the phenological stage, the contribution of root-derived C to total CO₂ emission from soil varied from 61 to 92% of total CO₂ evolved, including 4-23% attributed to rhizomicrobial respiration. While 81-91% of C substrates used for microbial growth in the root-free soil and rhizosphere came from SOM, the remaining 9-19% of C substrates utilized by the microbial biomass was attributable to rhizodeposition. The use of continuous isotopic labelling and physical separation of root-free and rhizosphere soil, combined with natural ¹³C abundance were effective in gaining new insight on soil and rhizosphere C-cycling.