Rice-wheat Rotation Fields Research Articles

Land-use intensification exerts a greater influence on soil microbial communities than seasonal variations in the Taihu Lake region, China

The Taihu Lake region has undergone intensive land-use conversions from natural wetlands (NW) to conventional rice-wheat rotation fields (RW) and further to greenhouse vegetable fields (GH). Nevertheless, the effects of these conversions on soil microbes, particularly in wetland ecosystem, are not well explicit. To explore the impact of land-use intensification on soil microbial communities, monthly soil samples were obtained from replicate plots representing three land-use types (NW, RW, and GH) in subtropical wetlands and then subjected to amplicon sequencing. Land-use intensification had direct effects on bacterial and fungal community composition, with a more pronounced impact on bacteria than on fungi. These changes in bacterial communities were closely correlated with variations in soil environmental variables, such as NO3−-N, pH, and electrical conductivity. Land-use intensification led to a decrease in bacterial deterministic processes, with an opposing trend observed in the fungal community. In addition, arable lands (RW and GH), which are affected by anthropogenic activities, exhibited more complex networks. Potential metabolic functional groups in GH had higher absolute abundance. Seasonal variations significantly influenced microbial diversity, composition, and potential metabolic functional groups within each land-use type, particularly in summer, although the magnitude of this impact was much smaller than the impact of land-use intensification. Our findings emphasize the importance of comprehending the ecological consequences of land-use intensification in wetlands for sustainable resource management and biodiversity conservation.

Long‐term successive biochar application increases plant lignin and microbial necromass accumulation but decreases their contributions to soil organic carbon in rice–wheat cropping system

AbstractBiochar application is widely recognized as an effective approach for increasing soil organic carbon (SOC) and mitigating climate change in agroecosystems. However, the effects of biochar application on net accumulations and relative contributions of different SOC sources remain unclear. Here, we explored the effects of biochar application on plant‐derived (PDC) and microbial necromass C (MNC) in a 10‐year experimental rice–wheat rotation field receiving four different intensities of biochar application (0, 2.25, 11.5, and 22.5 t ha−1 for each crop season), using phospholipid fatty acids (PLFAs), lignin phenols and amino sugars as biomarkers of microbial biomass, PDC and MNC, respectively. Our results showed that biochar application increased SOC content and stock by 32.6%–203% and 26.4%–145%, respectively. Higher biochar application (11.5 and 22.5 t ha−1) increased soil pH, total nitrogen (TN), total phosphorus (TP), SOC/TN, and root biomass. In addition, higher biochar application enhanced bacterial, fungal, and total microbial biomass. Plant lignin phenols and MNC contents significantly increased, whereas their contributions to SOC significantly decreased with the increase in biochar application rates due to the disproportionate increase in PDC and MNC, and SOC. Fungal necromass had a greater contribution to SOC than bacterial necromass. The fungal/bacterial necromass decreased from 2.56 to 2.26 with increasing biochar application rates, because of the higher abundances of bacteria than that of fungi as indicated by PLFAs under higher biochar application rates. Random forest analyses revealed that pH, TP, and SOC/TN were the main factors controlling plant lignin and MNC accumulation. Structural equation modeling revealed that biochar application increased lignin phenols by stimulating root biomass, whereas enhanced MNC accumulation was primarily from increased microbial biomass and lignin phenols. Overall, our findings suggest that biochar application increases the accumulation of the two SOC sources but decreases their contributions to SOC in paddy soils.

Effects of Straw Return Duration on Soil Carbon Fractions and Wheat Yield in Rice–Wheat Cropping System

In China’s subtropical rice–wheat cropping system, the changes in the soil organic carbon (SOC) pool due to long-term straw return and its connection with crop yield remain unclear. This study aims to provide insights into establishing a sensible straw return system by evaluating the differences in the distribution and variation rates of SOC, light fraction organic carbon (LFOC), heavy fraction organic carbon (HFOC), particulate organic carbon (POC), and mineral-associated organic carbon (MOC) in the 0–20 cm soil layer under different durations of straw return. Additionally, the study analyzes the relationship between the changes in SOC and its fractions and wheat yield. The experiment was conducted in 2019 in a rice–wheat rotation field with ten years of straw return treatments: no straw return (NR) or 1, 2, 3, 4, 5, 6, 7, 8, 9 year(s) of straw return (SR1–9), and an additional treatment in 2020 (10 years of straw return, SR10). The results revealed that with an increase in the duration of straw return, the contents of SOC, LFOC, HFOC, and POC gradually increased, showing the highest increments of 45.88%, 187.22%, 41.55%, 97.89%, and 28.21%, respectively, compared to the NR treatment. However, after eight years of straw return, the compound annual increase in soil organic carbon and its components was lower than in years 1–8, indicating a trend of diminishing increments. The SOC content and its variation were significantly correlated with the content and variation of LFOC, HFOC, POC, and MOC, with the highest sensitivity observed for the variation in LFOC, indicating the strong influence of the duration of straw return. The SOC and its fraction contents showed significant positive correlations with wheat yield, with the highest contribution to wheat yield increase attributed to an increase in LFOC content. In summary, straw return enhances the 0–20 cm deep soil carbon pool, with LFOC being the most sensitive indicator, reflecting the influence of the duration of straw return on soil carbon pools.

Effects of straw-returning combined with application of microbial inoculants on soil aggregates and related nutrients

We analyzed the particle size distribution of soil aggregates in 0-20 and 20-40 cm soil layers of rice-wheat rotation field based on a field plot test with two treatments, conventional straw returning (CK) and straw returning with the addition of straw decomposition promoting microbial inoculants (IT). We evaluated the water stability indices of soil aggregates (the number of soil water stable large aggregates R0.25, the average weight diameter MWD, and the geometric average diameter GMD), and measured the contents of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) in the soil aggregates of ＜0.053, 0.053-0.25, 0.25-1, ＞1 mm. The results showed that: 1) The number of aggregates ＜0.053, 0.053-0.25, ＞0.25 mm in the 0-20 and 20-40 cm soil layers under IT decreased by 10.0% and 6.8%, increased by 3.0% and 5.7%, and 17.9% and 26.1% compared with CK, respectively. IT effectively increased R0.25, MWD, and GMD by 26.4%, 20.0%, 18.2% and 18.2%, 10.5%, 10.0% in 0-20 and 20-40 cm soil, respectively. 2) Compared to CK, the TP content of 0.25-1 mm aggregates in 0-20 and 20-40 cm soil under IT was significantly increased by 40.3% and 37.5%, respectively, without difference in TN and SOC contents. There was no significant difference in nutrient contents of the other aggregates between the treatments. The contents of SOC and TN in large aggregates (>0.25 mm) were higher than those in silty aggregates (<0.053 mm). Compared to CK, the cumulative contribution rates of SOC, TN and TP of <0.053 mm aggregates under IT were decreased in two soil layers. There was no significant difference in the nutrient cumulative contribution rates of 0.053-0.25 mm aggregates between treatments. The cumulative contribution rates of SOC, TN, and TP of large aggregates (>0.25 mm) under IT were 32.1%, 19.6%, 52.8% and 22.8%, 11.8%, 42.9% higher than those under CK in 0-20 and 20-40 cm soils, respectively. 3) The number of <0.053 mm aggregates was significantly negatively correlated with SOC and TP contents, while that of 0.053-0.25 mm aggregates was negatively correlated with nutrient content. The number of large aggregates (>0.25 mm) were significantly positively correlated with SOC, TN, and TP contents. In conclusion, straw returning with microbial-inoculant addition could promote the formation of soil macroaggregates (>0.25 mm), and improve the water stability of soil aggregates, increasing nutrient contents in soil macroaggregates, with the nutrients transferring from silty aggregates to macroaggregates.

Straw return drives soil microbial community assemblage to change metabolic processes for soil quality amendment in a rice-wheat rotation system

Straw return has been widely implemented to sequester soil organic carbon (SOC) and enhance soil quality in rice-wheat cropping systems, however, the mechanism through which it influences microbial assemblages into mediating biochemical metabolic pathways in soil remains ambiguous. This study aimed at investigating the composition and assembly of soil microbial communities and the soil metabolome prevailing across four straw return practices (control: no straw return; RR: rice-straw return; WR: wheat-straw return; DR: both rice- and wheat-straw return) during wheat cultivation on a rice-wheat rotation field. Straw return primarily altered the abundance of lipids (LL), organic acids (OA), organic nitrogen compounds, and benzenoids, with DR having a greater impact on soil metabolites than WR or RR. Besides being predominantly present in the lipid (LM) and amino acid (AAM) metabolic pathways, these differentially expressed metabolites (DEMs) were also dispersed throughout the nucleotide, xenobiotic, and secondary metabolite pathways. Straw return substantially elevated homogeneous selection in the order DR > WR > RR > control. The links of taxa to LM and AAM occupied by 64.9% in the DEMs-microbial taxa correlation network, with majority of taxa being derived from the keystone modules of the microbial network. Furthermore, these taxa potentially modulated variations in microbiota assembly by 27–45.1%, soil metabolites by 13.3–70.7%, and soil nutrient properties by 24.8–49.6%; and their removal evidently mitigated the natural connectivity of the microbial network. Structural equation modeling depicted that straw return exerted positive effects on LL and OA by regulating microbial assembly. Therefore, our results demonstrated that straw return drive microbial community assembly to dominate the taxa mediating soil LM and AAM for enhancing SOC sequestration and soil quality.

First report ofFusarium meridionalecausing Fusarium head blight of wheat in Henan Province, China.

Fusarium graminearum and F. asiaticum have been found as a major cause of Fusarium head blight (FHB) of wheat (Triticum aestivum L.), especially in Henan Province of China (Zhang et al. 2014; Xu et al. 2021). In May 2021, a survey to determine the composition of Fusarium species infecting wheat heads was conducted in commercial fields in Henan. A total of 395 diseased spikes with premature whitening symptom were collected from 31 commercial fields in Henan. Symptomatic spikelets were excised, surface-sterilized for 10 s in 70% ethanol followed by 1 min in 3% sodium hypochlorite, rinsed three times with autoclaved distilled water, and then plated onto potato dextrose agar (PDA) medium. Isolated colonies that resembled Fusarium species were transferred to fresh PDA plates and purified using a single spore method. Species were identified based on sequence analysis of the translation elongation factor-1α (TEF) and trichothecene 3-Oacetyltransferase (Tri 101) gene (Proctor et al. 2009). The results indicated that F. graminearum (43.3%), F. asiaticum (47.8%), F. pseudograminearum (6.6%) were the main causal agents of FHB in Henan. However, nine isolates (2.3%) were found to be identical to F. meridionale by sequence comparison in GenBank, and eight isolates of which came from three fields with 1% to 2% diseased spikes near Reservoir Luhun (34.1255° N, 112.1111° E, altitude: 388 m above sea level), Songxian County of Henan. The isolates of F. meridionale were transferred onto carnation leaf agar (CLA) and incubated at 20℃ under black light blue illumination. Macroconidia were abundant, relatively slender, curved to almost straight, commonly six- to seven-septate, and 27.0 to 61.0 (average 44.0) μm × 3.2 to 6.8 (average 5.3) μm. Microconidia were not observed. The TEF sequences (Accession nos. OM460748 to OM460756) and the Tri 101 sequences (OM460759 to OM460767) of the nine isolates showed 99 to 100% similarity with the TEF and Tri 101 sequences of F. meridionale NRRL 28436 and NRRL 28723 (AF212435 and AF212436 (TEF); AF212582 and AF212683 (Tri 101)). To complete Koch's postulates, the pathogenicity of the fungus was tested by using the single floret inoculation method by injecting 20-μl conidial suspension (5 × 105 conidia per milliliter) into healthy inflorescences of wheat cultivar Bainong 207 at anthesis in the field. Another 30 healthy inflorescences were injected with sterile distilled water. The heads were covered with polyethylene bags that were removed after 2 days. Twenty days after inoculation, while control inflorescences were asymptomatic, the F.meridionale-inoculated inflorescences showed 12% bleached spikelets per spike. By using the methodology described above, the fungus was re-isolated from infected spikelets of inoculated wheat heads but not from the controls. Although F.meridionalehas frequently been reported in association with Fusariumear rot (FER) of maize in Chongqing City and Gansu Province (Zhang et al. 2014; Zhou et al. 2018), and riceFER in Sichuan Province (Dong et al. 2020), to our knowledge, this is the first report of F. meridionale from diseased wheat heads in Henan, China. Further investigation is needed to gain a better understanding of this species by collecting isolates from different cropping system in Henan, which maize-wheat and rice-wheat rotation fields have coexisted in the region.

RSARE: A physically-based vegetation index for estimating wheat green LAI to mitigate the impact of leaf chlorophyll content and residue-soil background

The green leaf area index (LAI) is an important structural parameter that can be used for monitoring plant transpiration and carbon cycling over large spatial extents. LAI is also critical for understanding biophysical processes and predicting crop productivity. However, the retrieval of wheat green LAI based on canopy reflectance is always affected by the background material. For example, in the Yangtze River Basin of China, rice residues are typically returned to rice–wheat rotation fields. Thus the background for observing wheat canopy varies from a mixture of traditional wheat-soil components to wheat-residue or wheat-residue-soil, posing a significant challenge for accurate LAI estimation at the early stages of crop growth. Furthermore, when current vegetation indices (VIs) are used to estimate LAI, leaf chlorophyll content (LCC) and field moisture are two additional confounding factors. To resolve these issues, we propose a novel residue-soil adjusted red edge difference index (RSARE), which draws on concepts from a simple physical algorithm, i.e., linear spectral mixture analysis (LSMA). The results of field, satellite and modeling experiments demonstrate that the RSARE-LAI model is generally not sensitive to LCC, or to variations in residue-soil and traditional soil backgrounds, and estimates LAI with high accuracy (RMSEval = 0.55, RRMSEval=20.71%). In comparison to traditional VI-LAI maps, RASRE-LAI maps are less sensitive to confounding factors associated with the background components, field moisture, LCC, and both inter-annual and geographical differences. This novel residue-soil background-resistant RSARE particularly improves the accuracy of wheat LAI estimation at low LAI levels, thus facilitating large-scale mapping of LAI in the early growing season.

Effects of tillage on soil organic carbon and crop yield under straw return

No-tillage with straw mulch is an effective way to achieve soil sustainability. However, the specific redistributions of soil organic carbon composition, stocks, and crop yield remain unclear. The hypothesis that no-tillage had a minor effect on soil organic carbon stocks in 0–50 cm but decreased crop yield compared to plow-tillage under straw return was tested in this study. Soils properties from two decades of rice-wheat rotation field experiment were related to the yield of rice and wheat depending on tillage: 1) plow-tillage without straw return, 2) no-tillage with straw mulch, and 3) plow-tillage with straw return. No-tillage with straw mulch had the highest mean weight diameter of soil aggregates, the contents of macro-aggregates (>0.25 mm) organic carbon, particulate organic carbon (>53 µm), and soil organic carbon across tillage practices in 0–5 cm. However, it decreased the contents of macro-aggregates (>0.25 mm) organic carbon in 5–15 cm, the particulate organic carbon in 5–50 cm, and soil organic carbon in 5–30 cm compared to plow tillage with straw return due to less carbon input from roots and straw. Organic carbon stocks in micro-aggregates (<0.25 mm) and mineral-associated organic matter (<53 µm) did not vary between tillage practices. Hence the soil organic carbon stocks of plow tillage without straw return (48 Mg ha−1) were similar to that of no-tillage with straw mulch (51 Mg ha−1) but lower than that of plow tillage with straw return (57 Mg ha−1) in 0–50 cm. The mean annual yields of wheat and rice under no-tillage with straw mulch (14 t ha−1) were lower than that of plow tillage without straw return (15 t ha−1) and plow tillage with straw return (16 t ha−1). No-tillage-induced high soil bulk densities limited the rice yields. The yield losses of no-tillage with straw mulch were higher in rice than in wheat. The rice yield losses of no-tillage with straw mulch compared to plow tillage with straw return increased with experiment duration due to decreased available nitrogen in 7–14 cm and phosphorus contents in 0–14 cm over time. Conclusively, plow-tillage is more efficient for carbon sequestration and yield increase than no-tillage under straw return in rice-wheat farming by increasing carbon input in deep soils, soil available nutrients, and decreasing soil compactness.

Modeling ammonia emissions and abatement potential from the rice-wheat rotation fields using the calibrated DNDC model: A case study in Shanghai, China

Abating ammonia (NH3) volatilization from intensive cropping system is of great importance for preventing the pollution of atmospheric environment and sustainable agriculture. Urea, a common nitrogen fertilizer, can convert to an unstable carbamate and further releases NH3 from the fields. However, few investigations of NH3 volatilization loss following fertilizer application to agricultural soils have been done using process-based models. In this study, a two-year field study was conducted to calibrate and evaluate the DeNitrification-DeComposition (DNDC) model for simulating NH3 volatilizations and crop production for the rice-wheat rotation system under four fertilizer regimes: control group without N fertilizer (CK), chemical fertilizer (CF), mixed application of inorganic and organic fertilizer (MF), and organic fertilizer (OF) treatments. The calibrated and modified DNDC performed effectively in simulating cumulative NH3 volatilization (coefficient of determination (R2) = 0.96 for rice and 0.92 for wheat field, relative deviation (RD) = −6.3% - 2.1% for rice and −22.5 to −4.6% for wheat) and grain yields for both crops (R2 = 0.94 for rice and 0.91 for wheat, RD of rice: 4.94%–9.59%; wheat: −1.84%–11.51%). The sensitivity analysis demonstrated that NH3 volatilization from rice-wheat rotation field was most sensitive to N application rate. Compared to CF and OF, MF treatments significantly mitigated NH3 volatilization and improved crop yield. Therefore, scenario simulation was conducted to confirm the optimal N application rate under MF. The results revealed that 250 kg N ha−1 and 112.5 kg N ha−1 in MF were recommended for rice- and wheat-growing, respectively. Compared to the current N fertilizer rate, the recommended rate saved 50 kg N ha−1 and mitigated NH3 volatilization loss by 28.31% from rice field and saved 37.5 kg N ha−1 and mitigated NH3 volatilization loss by 19.29% from wheat field. Overall, the calibrated DNDC model proved to be an effective tool to simulate NH3 volatilization from the rice-wheat rotation system and provides a promising fertilizer management strategy to decrease N input and mitigate NH3 volatilization.

Responses of soil microbial communities to manure and biochar in wheat cultivation of a rice-wheat rotation agroecosystem in East China

Soil contamination in agroecosystems remains an environmental problem of global concern. Biochar has been suggested as an organic amendment to alleviate soil pollution, sequester carbon, and improve fertility. Yet, information is still lacking on how bacterial and fungal communities in acidic bulk and rhizosphere soils respond to swine manure and its biochar. In this study, biochar and swine manure were applied at two rates of 1.5 and 3 t ha-1 in a rice-wheat rotation field study to assess how soil characteristics, especially pH and the availability of chemical elements, correlate to compositional variations of bacteria and fungi in bulk and rhizosphere soils. Our results showed that biochar and manure amendments at higher rates promoted bacterial richness index in bulk and rhizosphere soils through increasing soil pH and reducing As and Cu availability. For beta diversity, compared with As and Cu availability, soil pH had opposite effects on both bacterial and fungal communities. Specifically, biochar and swine manure application stimulated bacterial class of Gemmatimonadetes, Deltaproteobacteria, and Gammaproteobacteria via increasing pH and decreasing available chemical elements. Opposite trends of fungal community responding to biochar and manure was found. For example, biochar restrained the fungal class of Eurotiomycetes via decreasing As and Cu, but manure inhibited Leotiomycetes mainly due to an increase in soil pH and a decrease in soil dissolved organic C (DOC). These evidences suggest that both bacterial and fungal communities respond significantly to biochar and manure amendments in both bulk and rhizosphere soils, possibly due to their sensitive adaptation to soil environmental variations, such as pH levels and availability of chemical elements.

Seasonal dynamics of carbon dioxide and water fluxes in a rice-wheat rotation system in the Yangtze-Huaihe region of China

Investigating variations in the characteristics and influencing factors on carbon dioxide and water fluxes in cultivated lands are crucial for improving crop yield and water use efficiency. As a critical cropping system, rice-wheat rotations are widely distributed in South and East Asia. However, few studies have reported on the long-term changes in carbon dioxide and water fluxes in rice-wheat rotation systems using the eddy covariance method. In this study, a typical rice-wheat rotation field located in Anhui province, China, was chosen for our study. We analyzed seasonal dynamics and influencing factors of net ecosystem CO2 exchange (NEE), gross primary production (GPP), ecosystem respiration (Re), and evapotranspiration (ET) based on 5-year observations and boosted regression trees method. The main results were as follows: (1) daily NEE, GPP, Re, and ET displayed a bimodal curve during the year, with two peaks in the rice and wheat growing seasons, respectively. (2) The cumulative values of NEE were negative in the rice and wheat growing seasons, indicating the cultivated land acted as a carbon sink during the study period. Furthermore, cumulative values of NEE, GPP, Re, and ET were slightly higher in the rice-growing seasons than the wheat-growing seasons. (3) The relative importance analysis revealed the leaf area index (LAI) was the primary control factor of NEE and GPP, whereas temperature and LAI essentially controlled Re. In comparison, net radiation was the most crucial controller of ET. However, their contributions varied in different growing seasons. The findings have important implications for parameterizing crop models to improve simulating carbon and water fluxes for rice-wheat rotation systems.

Land use intensification significantly reduced CH4 emissions while increasing N2O emissions: Taihu Lake region, China

Land use intensification is becoming increasingly common in China’s Taihu Lake region because of economic interests. Frequent intensification is the conversion from natural wetlands (NW) to conventional rice-wheat rotation fields (RW), and from conventional RW to greenhouse vegetable fields (GH). However, the impact of land use intensification on greenhouse gas emissions in this area is still unknown. In our study, we chose typical NW, RW, and GH to compare and measure methane (CH4) and nitrous oxide (N2O) emissions from the three land use types through two-year in-situ field experiments. The results showed that land use intensification significantly reduced CH4 emissions, and NW, RW, and GH emissions were 970.66 ± 100.09, 896.71 ± 300.44, and −71.23 ± 63.62 kg C ha-1. Land use intensification significantly increased N2O emissions, and NW, RW, and GH emissions were 3.35 ± 0.44, 14.38 ± 4.09, and 81.62 ± 4.89 kg N ha-1. The global warming potential of CH4 and N2O under different land use types was in the order of RW (28,137.98 ± 9077.12 kg eq-CO2 ha-1) > NW (27,123.76 ± 2703.34 kg eq-CO2 ha-1) > GH (20,360.44 ± 2651.51 kg eq-CO2 ha-1). Overall, we observed that the comprehensive greenhouse effect of CH4 and N2O was enhanced by conversion from NW to RW and weakened by conversion from RW to GH. Our study provides a predictive understanding of how land use intensification changes greenhouse gas emissions in the Taihu Lake region and in regions with similar land use types.

Soil bacterial communities and co-occurrence changes associated with multi-nutrient cycling under rice-wheat rotation reclamation in coastal wetland

Coastal reclamation is a global threat to coastal wetland ecosystems, but the impacts of coastal reclamation on belowground biodiversity and their consequences for ecosystem nutrient cycling remain poorly understood. In this study, we examined soil bacterial communities, bacterial co-occurrence pattern and soil multi-nutrient cycling index after natural vegetated coastal wetlands were converted for 6-year and 30-year rice-wheat rotation fields. The soil multi-nutrient cycling index was higher in 6-year and 30-year rice-wheat rotation fields than natural vegetated coastal wetlands. For bacterial community, rice-wheat rotation reclamation increased soil bacterial alpha diversity but decreased beta diversity and co-occurrence network complexity than those in the vegetated coastal wetland. Rice-wheat rotation reclamation also changed soil bacterial community assemblages by increasing the relative abundance of Actinobacteriota and Desulfobacterota while decreasing the relative abundance of Alphaproteobacteria and Gammproteobacteria. Changes in the relative importance of dispersal limitation and homogenizing dispersal mainly controlled the variations in soil bacterial community assemblages. Soil bacterial community variations induced by rice-wheat reclamation were strongly related to soil multi-nutrient cycling, especially bacterial connection complexity. Our results provide experimental support that belowground microbial interactions are crucial for maintaining ecosystem functioning.

Improvement of P Use Efficiency and P Balance of Rice–Wheat Rotation System According to the Long-Term Field Experiments in the Taihu Lake Basin

Phosphorus (P) accumulation in rice–wheat rotation fields around the Yangtze River delta have been enriched during the last decades. To protect the environment and save P resources, we conducted field experiments to optimize the P application scheme. First, one field experiment was designed as a series of P fertilizer application doses of 0–100 kg P2O5 hm−2. Grain yield and P uptake by crops were analyzed to calculate P surplus and P use efficiency. Soil P fractions were extracted and tested. According to the P balance, we optimized fertilization by reducing the chemical P amount, which was used by local farmers; furthermore, we substituted chemical P with organic fertilizer. To verify these management strategies, another field experiment was conducted with five treatments: no N, P, or K fertilizer (CK); only no P fertilizer (NK); farmers’ fertilization of P (90 kg P2O5 hm−2) (FFP); reducing 20% P (FFP-20%P); and reducing 20% P and replacing 20% P by manure (FFPM-36%P). The grain yield was enhanced by increased P fertilizer and reached a constant level after 75 kg P2O5 hm−2. Moreover, the annual P surplus was balanced around the input of 150 kg P2O5 hm−2. Accordingly, by optimizing fertilization (FFP-20%P) and further replacing manure (FFPM-36%P), we also achieved crop yield equivalent to that of FFP treatment (90 kg P2O5 hm−2). Thus, the 72–75 kg P2O5 hm−2 application rate is a threshold for the production of rice and wheat and P balance. Total P content in soil was enhanced by increased input of P fertilizer and mainly divided into labile Pi and middle stable Pi fractions. Soil Olsen-P content increased by P fertilization accordingly, while the content of organic P and stable P content was relatively constant. Reducing P fertilizer by 20% had similar results for soil P fractions when compared with farmers’ P fertilization treatment. Therefore, reducing at least 20% current input of P by farmers (annual 180 kg P2O5 hm−2) according to the balance of P surplus in rice and wheat rotation systems is an imperative measure to guarantee crop production with enhanced P use efficiency, and meanwhile, it can alleviate environmental risk.

The Classification of Farming Progress in Rice–Wheat Rotation Fields Based on UAV RGB Images and the Regional Mean Model

Extraction of farming progress information in rice–wheat rotation regions is an important topic in smart field research. In this study, a new method for the classification of farming progress types using unmanned aerial vehicle (UAV) RGB images and the proposed regional mean (RM) model is presented. First, RGB information was extracted from the images to create and select the optimal color indices. After index classification, we compared the brightness reflection of the corresponding grayscale map, the classification interval, and the standard deviation of each farming progress type. These comparisons showed that the optimal classification color indices were the normalized red–blue difference index (NRBDI), the normalized green–blue difference index (NGBDI), and the modified red–blue difference index (MRBDI). Second, the RM model was built according to the whole-field farming progress classification requirements to achieve the final classification. We verified the model accuracy, and the Kappa coefficients obtained by combining the NRBDI, NGBDI, and MRBDI with the RM model were 0.86, 0.82, and 0.88, respectively. The proposed method was then applied to predict UAV RGB images of unharvested wheat, harvested wheat, and tilled and irrigated fields. The results were compared with those obtained with traditional machine learning methods, that is, the support vector machine, maximum likelihood classification, and random forest methods. The NRBDI, NGBDI, and MRBDI were combined with the RM model to monitor farming progress of ground truth ROIs, and the Kappa coefficients obtained were 0.9134, 0.8738, and 0.9179, respectively, while traditional machine learning methods all produced a Kappa coefficient less than 0.7. The results indicate a significantly higher accuracy of the proposed method than those of the traditional machine learning classification methods for the identification of farming progress type. The proposed work provides an important reference for the application of UAV to the field classification of progress types.

Sensitivity of soil aggregation to soil organic carbon fractions under land-use conversion from rice to organic vegetable cultivation

Conversion of rice-wheat rotation (RWR) to conventional vegetable cultivation, especially vegetable cultivation in plastic sheds, readily causes soil structure degradation, and organic manure is commonly applied to mitigate such degradation. Although many studies have focused on changes in SOC properties or structural soil parameters, additional details about the changes, such as the relationships between SOC fractions and soil aggregates during unique land-use changes, are unknown. Here, we studied the changes in soil aggregation, and SOC parameters in 14-year-old plastic-shed vegetable fields (PVFs) and open-air vegetable fields (OVFs) covered with organic manure, with adjacent RWR fields serving as a control. The vegetable fields were converted from RWR fields. SOC fractions were analyzed by using 13C solid-state NMR spectroscopy, and the aggregate classes were divided as follows: large macro-aggregates (>2 mm), small macro-aggregates (2–0.25 mm), micro-aggregates (0.25–0.053 mm), and silt and clay particles (<0.053 mm). PVFs had higher SOC and MBC than OVFs and RWR fields. In terms of the SOC fractions, the proportion of O-alkyl C decreased as RWR (53.90%) > PVF (43.21%) > OVF (35.37%), in contrast to the trends in the carbonyl C and aromatic C fractions. The aggregate-associated C, especially that associated with large macro-aggregates and micro-aggregates, in the PVFs was highest among the treatments. The O-alkyl C fraction in large macro-aggregates decreased in the order RWR (44.67%) > OVF (35.76%) > PVF (32.40%), consistent with the results for micro-aggregates. Furthermore, there was a significant (P ≤ 0.05) positive relationship between macro-aggregates (>0.25 mm) and the active C fractions, in contrast to the relationships with stable fractions. It suggested that plastic-shed cultivation might have a more positive effect than open-air cultivation on soil structure and the carbon stock when large amounts of organic manure are applied; moreover, the SOC quantity and quality affect soil aggregation differently.

Assessment of soil aggregation properties after conversion from rice to greenhouse organic cultivation on SOC controlling mechanism

Organic manure is beneficial for macro-aggregate formation and soil organic carbon (SOC), but how SOC change in aggregate fractions in time-series is still uncertain. Moreover, greenhouse systems converted from cereal fields quickly faced soil degradation. Thus, the role of organic manure here should be discussed. The main objectives of this study were to determine the change of SOC fractions in bulk soil and aggregation level affected by long-term organic manure application. Using 13C solid-state nuclear magnetic resonance (NMR) spectroscopy, we investigated the SOC and its fraction changes within bulk soil and aggregate fractions under 1-year, 9-year, and 14-year organic greenhouse vegetable cultivation, and we also analysed the soil properties of rice-wheat rotation (RWR) fields as the control. Soil aggregate samples were wet sieved into large macro-aggregates (> 2 mm), small macro-aggregates (2–0.25 mm), micro-aggregates (0.053–0.25 mm), and silt and clay particles (<0.053 mm). The proportion of large macro-aggregates increased significantly (P ≤ 0.05) from 1.68 to 7.76% during the 14-year organic cultivation period. Similar trends could also be found in SOC and its fractions. Specifically, the O-alkyl C increased fastest by 14.5 g kg−1 over these years, but its proportion decreased. The change in aggregate associated C was concentrated on large macro-aggregates and micro-aggregates. Pearson’s correlation suggested that there was a non-significant relation between SOC and soil aggregation, while the soil aggregate associated C had a significantly positive relationship with SOC and its fraction amounts. However, the proportion of O-alkyl C was less; the aggregation associated C was higher. This study has shown the different effects of SOC fractions on soil aggregation and aggregate associated C in organic greenhouse vegetable fields. The stable C fractions might show contribution in soil aggregate associated C than active C fractions. It might improve our knowledge about how organic manure may influence soil aggregation by increasing SOC after a long-term greenhouse vegetable plantation.

Effects of Urbanization on Watershed Evapotranspiration and Its Components in Southern China

Understanding the effects of land use change on evapotranspiration (ET) and its partitioning to transpiration and evaporation is important for accurately evaluating the likely environmental impacts on watershed water supply, climate moderation, and other ecosystem services (e.g., carbon sequestration and biodiversity). This study used a distributed hydrologic model, MIKE SHE, to partition evapotranspiration into soil evaporation, transpiration, ponded water evaporation, and interception, and examined how the ET partitions affected the water balance in the Qinhuai River Basin from 2000 to 2013. Simulated daily ET was compared to measurements at an eddy flux research site during 2016–2017 (R2 = 0.72). Degradation in rice-wheat rotation fields and expansion of impervious surfaces impacted not only total watershed evapotranspiration, which showed a significant downward trend (p &lt; 0.05), but also its partitioning. A significant (p &lt; 0.01) decrease in transpiration was detected. Ponded water evaporation was the only ET partition that exhibited a significant positive trend (p &lt; 0.05). We concluded that the reduced transpiration as a result of land use and land cover change was the primary factor driving the variation of watershed scale evapotranspiration. In addition, there was an increase in annual water yield (23%) as a response to significant reduction in ET (7%) due to a 175% expansion of urban area in the study watershed. Our study provided insights to the mechanisms of land surface–water cycle interaction and better understanding of the effects of land use change on urban micro-climate such as “urban dry island” and “urban heat island” effects.

Relationship between macropores and soil organic carbon fractions under long‐term organic manure application

AbstractGreenhouse vegetable production systems converted from cereal fields often experience a decrease in soil aeration due to intensive land use. Organic fertilization is a method commonly used to solve this problem. Although previous studies have considered soil organic carbon (SOC) or soil pores under organic cultivation, the relationship between SOC fractions and soil macropores is unknown. This study analysed the long‐term progressive variations in soil macropore (&gt;50 μm) and SOC fraction characteristics in organic greenhouse vegetable plantations (1‐year‐old, 9‐year‐old, and 14‐year‐old fields) converted from rice–wheat rotation fields and established the relationship with X‐ray computed tomography scanning and 13C solid‐state nuclear magnetic resonance spectroscopy. The results suggested that the total macroporosity increased from 7.95% to 16.36% throughout the whole organic plantation, and a similar trend occurred in small (50–500 μm) and medium (500–1,000 μm) macroporosity. Besides, the total SOC content and the contents of its fractions increased with organic planting time. With the total macroporosity exhibiting significant associations with SOC (r = .81**), the relative proportion of O–alkyl C (r = −.94**), and the relative proportion of aromatic C (r = .89**), a lower O–alkyl C proportion of SOC might help to increase macroporosity. A similar relationship was found between the relative proportion of SOC fractions and the macroporosity. The O–alkyl C proportion had significantly negative relations with aromatic C (r = −.94**) and SOC (r = .84**). Overall, both SOC and its fractions can help to improve soil macroporosity, but the effects of different fractions on these pores vary.

Effects of six-year biochar amendment on soil aggregation, crop growth, and nitrogen and phosphorus use efficiencies in a rice-wheat rotation

Abstract The effective use of biochar as biomass recycling in agriculture to improve both crop production and environmental performance requires an understanding of its effects on soil nutrients, i.e., nitrogen (N) and phosphorus (P) and soil structure and soil carbon stocks. Thus, crop yield, soil aggregate stability and N and P use efficiencies (NUE and PUE, respectively) were investigated in a rice-wheat rotation field experiment 6 years after biochar amendment (20 and 40 t ha−1). Biochar application increased crop (rice and wheat) root (by 3–19%), straw (by 10–19%) and grain (by 10–16%) biomasses, as well as grain NUE (by 20–53%) and PUE (by 38–230%) compared with N fertilization only, whereas decreases in the N stocks in the root (2–5%), straw (13–17%) and grain (0–2%) occurred after biochar addition in the rice growth stage. No differences were observed between 20 and 40 t ha−1 biochar with respect to root, straw and grain biomasses as well as NUE and PUE. Biochar improved soil organic carbon (by 26–53%), total N (by 14–16%) and P (by 6–19%) compared to N fertilization only and positively affected the 250–2000 μm aggregate class (by 87–93%) and aggregate stability (by 43–48%). Based on structural equation modeling (SEM), this additional carbon affected aggregate stability by improving the aggregate structure, whereas P affected aggregate stability directly in biochar-fertilized soils. Improvement of soil carbon stocks and nutrient pools (i.e., N, P), promoted root growth, uptake of N and P fertilizers and crop production. Thus, biochar application is an effective strategy to increase crop yield, even in the long-term, and is connected not only with the improvement of soil structure and carbon stocks but also with increases in nutrient use efficiency.