Biology and Fertility of Soils最新文献

Abstract

Alternate wetting and drying (AWD) is an effective water-saving practice for rice cultivation that may however promote nitrogen (N) losses compared to continuous flooding (CF). The interaction between water, crop residue and N fertilization management can influence the contribution of different N sources to plant uptake. We hypothesized that microbial processes driving the source-differentiated N supply for rice uptake during the early growth stages will depend on the interaction between water management, the timing of straw incorporation with respect to flooding and the temporal distribution of mineral N application. Rice was grown for 60 days in mesocosm experiment involving a factorial design with (i) two water regimes (CF vs. AWD) and (ii) three straw and fertilizer managements, during which soil N, porewater chemistry, plant growth and N uptake were evaluated. Source partitioning of plant N between fertilizer-, straw- and soil-derived N was achieved by means of a dual-stable isotope ¹⁵N tracing approach. Although AWD reduced total N uptake by about 4–25% with respect to CF, this could only be partly attributed to a lower uptake of fertilizer-N (and lower fertilizer-N use efficiency), suggesting that other N sources were affected by water management. Our findings evidence how the interaction between soil redox conditions and the availability of labile C and inorganic N strongly determined the supply of soil-derived N through microbial feedback and priming responses. Although incorporated straw contributed only minimally to rice N, it represented the primary driver controlling plant N nutrition through these microbial responses. These insights may contribute to identify suitable fertilization practices that favour plant N uptake during the early stages of rice growth under AWD.

Immobilization of plant growth-promoting rhizobacteria in biodegradable polymeric matrices has the potential to improve crop yield and overcome rhizosphere challenges. Previously, we demonstrated that beads prepared from blends of chitosan/starch are useful carriers for bacterial consortia applied to maize seeds, increasing seedlings growth. This work aimed to examine the application of Azospirillum argentinense and Pseudomonas rhodesiae co-immobilized in chitosan/starch beads as inoculants in late-sown maize crops under different environments and agronomic managements. We compared the ability to promote maize crop yield of co-immobilized bacteria with their liquid counterparts. We also analyzed if co-immobilized bacteria could compensate yield for the decline in resource availability caused by high-density sown. Our results revealed that in environments with limiting edaphic-climatic conditions for potential crop growth, maize inoculation with liquid formulations exhibited yield instability and unpredictability. Furthermore, when resources were limited by high plant density, only bead mediated inoculation maintained crop yield. The component responsible for the increase in yield caused by inoculation also varied depending on the environment. The weight of thousand grains explained the increase in yield at high yielding potential environment, whereas the number of grains explained the increase at lower yielding environment. The evidence collected here demonstrates that chitosan/starch beads are suitable for delivery of bacteria consortia as inoculants and more efficient than liquid inoculation, broadening the range of inoculant applications to diverse geographic areas and crop management.

Whether arbuscular mycorrhizal fungi (AMF) inoculation promotes soil C sequestration is largely unknown. Here, meta-analysis and logistic regression were applied to study the ecological effects of AMF inoculation on soil organic C (SOC) turnover and plant growth under different inoculation manipulations, plant traits, and soil conditions. Results showed that AMF inoculation generally increased SOC stock and plant biomass accumulation. Soil sterilization, unsterilized inoculum wash (a filtrate of mycorrhizal inoculum excluding AMF) addition in non-mycorrhizal treatments, experimental type, and inoculated AMF species influenced soil microbial biomass C (MBC) but had no impact on SOC turnover. Plant root system, initial SOC content, and soil pH were the key factors that influenced the AMF-mediated SOC turnover. AMF inoculation in fertile or acidic soils might deplete SOC. The symbiosis between tap-rooted plants and AMF was more likely to sequestrate C into the soil compared to fibrous-rooted plants. Moreover, plant total dry biomass largely relied on its own photosynthetic pathway although AMF was introduced. Collectively, our results suggest that AMF inoculation is a promising approach for soil C sequestration.

A 163-day decomposition experiment with ¹³C-enriched leaf litter of Populus davidiana (low quality, with low N content, high C:N and high lignin content) and Quercus wutaishanica (high quality, with high N content, low C:N and low lignin content) was conducted to investigate the effects of litter quality on the microbial contribution to soil organic C (SOC). We used stable isotope probing (SIP) technology of phospholipid fatty acid (PLFA) and amino sugar, determined soil enzyme activities, and microbial C use efficiency (CUE) to study the microbial contribution to SOC formation as affected by litter quality. Gram-positive (G +) and Gram-negative (G −) bacteria rapidly assimilated the readily available C of high- and low-quality litter, whereas fungi selectively utilized more recalcitrant compounds. The ratio of ¹³C-fungal to ¹³C-bacterial necromass increased and then leveled off until the end of the incubation for both litters. Therefore, litter-derived C was first utilized by bacteria, then allocated presumably by the consumption of bacterial necromass to fungi, and, at the end, the litter C was mainly stabilized as fungal necromass. The addition of high-quality litter led to higher total necromass and SOC in comparison to the addition of low-quality litter. Likely this difference depended on the higher availability of easily available C compounds in the Q. wutaishanica than in P. davidiana litters. The efficiency of SOC formation, determined by the percentage of SOC content gain divided by the litter C content loss, correlated with the microbial incorporation of P. davidiana litter-derived ¹³C into PLFAs and amino sugars. However, it increased sharply in the late phases of Q. wutaishanica litter decomposition despite the decreased ¹³C incorporation in PLFAs and amino sugars, suggesting the dominance of physical litter C stabilization. Compared to the high-quality litter, the low-quality litter induced lower but steadier necromass accumulation, thus increasing the SOC content in the long term. Litter quality, litter-derived ¹³C in PLFAs, and microbial CUE are the main drivers of litter-derived C use pathways. Our findings underpin the microbial C pump-regulated SOC formation, whereby differences in litter quality shape the composition of main microbial groups, leading to differences in enzyme activities and CUE, which determine necromass turnover and thus SOC formation.

Acid-sulphate sugarcane soils in the subtropics are known hot-spots for nitrous oxide (N₂O) emissions, yet the reduction of reactive N₂O to non-reactive dinitrogen (N₂) via specific pathways remains a major uncertainty for nitrogen (N) cycling and loss from these soils. This study investigated the magnitude and the N₂O:N₂ partitioning of N₂O and N₂ losses from a subtropical acid-sulphate soil under sugarcane production using the ¹⁵N gas flux method, establishing the contribution of hybrid (co- and chemo-denitrification) and heterotrophic denitrification to N₂O and N₂ losses. Soils were fertilised with potassium nitrate, equivalent to 25 and 50 kg N ha⁻¹, watered close to saturation then incubated over 30 days. An innovative, fully automated incubation system coupled to an isotope-ratio mass-spectrometer enabled real time analysis of ¹⁵N₂O and ¹⁵N₂ at sub-diel resolution. Peak losses of N₂O and N₂ reached 6.5 kg N ha⁻¹ day⁻¹, totalling > 50 kg of N₂O+N₂-N ha⁻¹. Emissions were dominated by N₂, accounting for more than 57% of N₂O+N₂ losses, demonstrating that the reduction of N₂O to N₂ proceeded even under highly acidic conditions. Over 40% of N₂O, but only 2% of N₂ emissions, were produced via hybrid pathways. These findings demonstrate hybrid pathways are generally limited to N₂O production, likely driven by high organic matter content and low soil pH, promoting both biotic, and abiotic nitrosation. Regardless of the underlying process, the magnitude of the N₂O emissions demonstrates the environmental, but also the potential agronomic significance, of hybrid pathways of N₂O formation for N loss from fertilised acid-sulphate soils.

Surface drip fertigation has demonstrated promising results regarding the mitigation of nitrous oxide (N₂O) emissions. The use of subsurface irrigation may offer the possibility of reducing these emissions further due to the modification of the soil moisture profile and N allocation, both of which affect the biochemical processes leading to N₂O fluxes. However, the mitigation potential of subsurface irrigation combined with different mineral nitrogen (N) fertilizers (ammonium or nitrate-based, use of nitrification inhibitors) still needs to be evaluated. To respond to this need, a 2-year field experiment was set up in central Spain to test two different drip-fertigation systems (surface and subsurface at 30 cm depth) and four N fertilization treatments (control, calcium nitrate, and ammonium sulfate with or without the nitrification inhibitor 3,4-dimethylpyrazole phosphate, DMPP) in an irrigated maize (Zea mays L.) crop. Nitrous oxide emissions, mineral N concentrations (ammonium, NH₄⁺, and nitrate, NO₃⁻), and abundance of key N genes involved in nitrification and denitrification processes were measured in two soil layers (0–20 and 20–40 cm). Regardless of the irrigation system, ammonium sulfate gave the highest cumulative N₂O losses in both campaigns, while calcium nitrate and the use of DMPP were the most effective strategies to abate N₂O fluxes in the first and second years, respectively. Differences between irrigation systems were not statistically significant for cumulative N₂O emissions, despite the clear effect on topsoil mineral N (higher NH₄⁺ and NO₃⁻ concentrations in surface and subsurface drip, respectively). Nitrous oxide emissions were positively correlated with soil NH₄⁺ concentrations. Gene abundances were not a trustworthy predictor of N₂O losses in the 1st year, although a clear inhibitory effect of fertilization on microbial communities (i.e., ammonia oxidizers, nitrite reducers, and N₂O reducers) was observed during this campaign. During the second year, nitrifying and denitrifying genes were affected by irrigation (with higher abundances in the 20–40 cm layer in subsurface than in surface drip) and by the addition of DMPP (which had a detrimental effect on gene abundances in both irrigation systems that disappeared after the fertigation period). In conclusion, the use of DMPP or calcium nitrate instead of ammonium sulfate may enhance the chances for an additional mitigation in both surface and subsurface irrigation systems.

Soil freeze-thaw cycles affect N₂O fluxes in high- and mid-latitude regions, but understanding microbial processes behind N₂O will help clarify the long-term impact of freeze-thaw on climate change. The aim of this study was to investigate the impacts of freeze-thaw cycles on microbial abundances and N₂O emissions in a hemi-boreal drained peatland forest. The soil freeze-thaw experiment involved artificial heating to thaw the topsoil after freezing. Results showed that thawing of the 5 cm topsoil increased soil water content (SWC) and N₂O emissions. Microbial analysis demonstrated that the abundance of soil prokaryotes increased with thawing. N₂O emissions were negatively correlated with NH₄⁺-N while ammonia-oxidizing archaea and bacteria, including complete ammonia oxidizers, increased their abundance. This indicates a potential nitrification pathway. The abundance of nitrite reductase genes (nirK and nirS) showed a positive correlation with N₂O fluxes, while nosZ genes did not increase. The results provide an insight into the impact of soil freeze-thaw cycles on N₂O fluxes and the underlying microbial processes. The dynamics of SWC during the thawing period were the most direct driver of the increase in N₂O emissions. Incomplete denitrification was the dominant process for the N₂O emissions during the thaw. More than 80% of produced N₂O was denitrified to inert N₂, as shown by high potential N₂ emissions. The frequency of freeze-thaw events is expected to increase due to climate change; therefore, determining the underlying microbial processes of the N₂O emissions under freeze-thaw is of great importance in predicting possible impacts of climate change in forests.

Soil microbial nitrate (NO₃⁻) immobilization plays a vital role in enhancing the nitrogen (N) retention in the subtropical montane agricultural landscapes. However, how and why the potential microbial NO₃⁻ immobilization and the relative contribution of fungi and bacteria vary across different land use types remain still unclear in the subtropical mosaic montane agricultural landscapes. Thus, in the present study, soil gross microbial NO₃⁻ immobilization rates as well as the respective contribution of fungi and bacteria were determined throughout the whole soil profiles for three land use types (woodland, orchard, and cropland) by using the ¹⁵N tracing and amino sugar–based stable isotope probing (Amino sugars-SIP) techniques. The soil gross microbial NO₃⁻ immobilization rates in woodland soils were significantly higher than those in cropland and orchard soils across different soil layers (p < 0.05), and those of topsoil were significantly higher than those for subsoils (e.g., 20–40 cm) across different land use types (p < 0.05). Soil microbial biomass C (MBC) and N (MBN), organic C (SOC), total N (TN), and dissolved organic C (DOC) contents and C/N ratios were closely associated to gross microbial NO₃⁻ immobilization rates. Fungi played a greater role than bacteria in immobilizing soil NO₃⁻ in woodland and orchard soils, but the opposite occurred in cropland soils that over 85% of the variations in fungal and bacterial NO₃⁻ immobilization rates could be explained by their respective phospholipid fatty acid–derived (PLFA-derived) biomass. The present study indicated that afforestation may be effective to enhance soil NO₃⁻ retention in alkaline soils, thereby likely decreasing the risk of NO₃⁻ losses in subtropical mosaic montane agricultural landscapes through enhancing the soil NO₃⁻ immobilization by both fungi and bacteria.

Reversing land management from no-tillage to conventional tillage (tillage reversal, TR) may markedly alter soil greenhouse gas (GHG) emissions in soils with differing fertility levels. We studied the impact of TR and nitrogen (N) fertilization on CO₂ (total CO₂ flux and its components), N₂O and CH₄ fluxes, and area- and yield-scaled GHG fluxes over two growing seasons in central Alberta, Canada. A split-plot design was used with two levels of N, 0 (N0) vs. 100 kg N ha^− 1 yr^− 1 (N100), and tillage, long-term no-tillage (NT) vs. TR, treatments. The TR treatment increased total CO₂ fluxes (R_t), mainly attributed to the increased CO₂ production from microbial activity (R_h), with the R_h/R_t ratio ranging between 52 and 61% in this study. The area-scaled GHG fluxes ranged from 3.10 to 4.50 Mg CO₂-C eq. ha^− 1, while the yield-scaled GHG fluxes ranged from 1.36 to 5.84 kg CO₂-C eq. kg^− 1 grain. The area-scaled GHG fluxes were 0.74 Mg CO₂-C eq. ha^− 1 higher in the TR than in the NT treatment, and 14.7% higher in the N100 than in the N0 treatment. Nitrogen fertilization did not affect the yield-scaled GHG fluxes; however, the TR treatment lowered the yield-scaled GHG fluxes due to the significantly increased crop yield. Therefore, management decisions will have to consider whether the objective is to reduce total GHG emissions on an area basis or to minimize GHG emissions per unit crop yield. Our study shows that periodic tillage of long-term NT soils increased yield and reduced yield-scaled GHG emissions, suggesting that tillage reversal is practical if the management objective is to maximize yield and minimize GHG emissions per unit crop yield.