Fertilization is an effective technique to improve soil fertility and increase crop yield. The long-term effects of different fertilizers on soil considerably vary. Over 38 consecutive years of different fertilization positioning experiments in a double cropping rice field of Qiyang Red Soil Experimental Station, seven different fertilization treatments including CK (no fertilization), NPK (nitrogen, phosphorus, and potassium fertilizer), M (cow manure), NPKM (nitrogen, phosphorus, and potassium with cow manure), NPM (nitrogen and phosphorus with cow manure), NKM (nitrogen and potassium with cow manure), and PKM (phosphorus and potassium with cow manure) were applied to study the effects on rice yield, soil fertility, and nutrient apparent balance in a paddy field. The results showed that the annual average yields of rice in NPKM, NPM, NKM, PKM, M, NPK and CK treatments ranged from 6 214 to 11 562 kg ha^–1. Yields under longterm organic and inorganic treatments (NPKM, NPM, NKM and PKM) were 22.58, 15.35, 10.53 and 4.41%, respectively, greater than under the NPK treatment. Soil organic carbon (SOC), total nitrogen (TN), available nitrogen (AN) and available potassium (AK) concentration with long-term organic and inorganic treatment (NPKM, NPM, NKM and PKM) were significantly higher than in inorganic fertilizer (NPK) treatments. Soil total phosphorus (TP) and available phosphorus (AP) contentration with organic fertilizer combined with inorganic N and P fertilizer treatment (NPKM, NPM and PKM) were significantly higher than with inorganic fertilizer alone (NPK treatments). The average annual rice yield (11 562 kg ha^–1), SOC (20.88 g kg^–1), TN (2.30 g kg^–1), TP (0.95 g kg^–1), TK (22.50 g kg^–1) and AP (38.94 mg kg^–1) concentrations were the highest in the NPKM treatment. The soil AN concentration (152.40 mg kg^–1) and AK contentration (151.00 mg kg^–1) were the highest in the NKM treatment. N and P application led to a surplus of nitrogen and phosphorus in the soil, but NPKM treatment effectively reduced the surplus compared with other treatments. Soils under all treatments were deficient in potassium. Correlation analysis showed that SOC, TN, AN, TP, and AP contentration was significantly correlated with rice yield; the correlation coefficients were 0.428, 0.496, 0.518, 0.501, and 0.438, respectively. This study showed that the combined application of N, P, and K with cow manure had important effects on rice yield and soil fertility, but balanced application of N, P, and K with cow manure was required.

Dissimilatory Fe(III) reduction is an important process in the geochemical cycle of iron in anoxic environment.  As the main products of dissimilatory iron reduction, the Fe(II) species accumulation could indicate the reduction ability.  The effects of different green manures on Fe(III) reduction in paddy soil were explored based on a 31-year rice-rice-winter green manure cropping experiment.  Four treatments were involved, i.e., rice-rice-milk vetch (RRV), rice-rice-rape (RRP), rice-rice-ryegrass (RRG) and rice-rice-winter fallow (RRF).  Soils were sampled at flowering stage of milk vetch and rape (S1), before transplantation (S2), at tillering (S3), jointing (S4), and mature (S5) stages of the early rice, and after the harvest of the late rice (S6).  The contents of TFe_HCl (HCl-extractable total Fe), Fe(II)_HCl (HCl-extractable Fe(II) species) and Fe(III)_HCl (HCl-extractable Fe(III) species) were measured.  The correlations among those Fe species with selected soil environmental factors and the dynamic characteristics of Fe(II)_HCl accumulation were investigated.  The results showed that TFe_HCl in RRF was significantly higher than those in the green manure treatments at most of the sampling stages.  Fe(II)_HCl increased rapidly after the incorporation of green manures in all treatments and kept rising with the growth of early rice.  Fe(II)_HCl in RRG was quite different from those in other treatments, i.e., it reached the highest at the S2 stage, then increased slowly and became the lowest one at the S4 and S5 stages.  Fe(III)_HCl showed oppositely, and Fe(II)_HCl/Fe(III)_HCl performed similarly to Fe(II)_HCl.  The maximum accumulation potential of Fe(II)_HCl was significantly higher in RRF, while the highest maximum reaction rate of Fe(II)_HCl accumulation appeared in RRG.  Significant correlations were found between the indexes of Fe(II)_HCl accumulation and soil pH, oxidation-reduction potential (Eh) and total organic acids, respectively.  In together, we found that long-term application of green manures decreased the TFe_HCl in red paddy soils, but promoted the ability of Fe(III) reduction, especially the ryegrass; Fe(II)_HCl increased along with the growth of rice and was affected by soil conditions and environmental factors, especially the water and redox ability.

On the basis of a long-term (30 years) field experiment that involved four rotation systems, rice-rice-winter fallow (RRF), rice-rice-ryegrass (RRG), rice-rice-rape (RRP), and rice-rice-milk vetch (RRV), this study described the effects of green manure on the microbial communities in the red paddy soils using 454 pyrosequencing for the 16S rRNA gene. The Chao1 richness and non-parametric Shannon’s index increased in all soil samples that received green manure treatments. The communities’ structures with the green manure applications were significantly dissimilar from that under the winter fallow. Using Metastats tests, many genera in the RRG, RRP and RRV soils were significantly different from those in the RRF soil, including a number of genera that functioned in the nitrogen and sulfur cycles. Analyses of the genera with these functions revealed the shifts in microbial ecosystem functions after long-term green manuring. Changes in the microbial communities increased the ammonium supply and decreased the soil acidification in green-manure-amended soils. Together, these data suggested powerful effects of green manure on both the microbial communities and the biogeochemical cycle driven by the shifts in bacterial functional groups.

Soil physical properties are important indicators of the potential for agricultural production. Our objective was to evaluate the effects of long-term inputs of green manures on physical properties of a reddish paddy soil (Fe-Typic Hapli-Stagnic Anthrosols) under a double cropping system. The common cropping pattern before the study was early-late rice-fallow (winter). The field treatments included rice-rice-fallow (R-R-WF), rice-rice-rape (R-R-RP), rice-rice-Chinese milk vetch (RR- MV), and rice-rice-ryegrass (R-R-RG). The rape, Chinese milk vetch and ryegrass were all incorporated as green manures 15 d before early rice transplanting during the following year. The soil bulk density in all green manure treatments was significantly reduced compared with the winter fallow treatment. Soil porosity with green manure applications was significantly higher than that under the winter fallow. The green manure treatments had higher 0.25-5 mm water stable aggregates and aggregates stabilities in the plow layer (0-15 cm depth) compared with the fallow treatment. The mean weight diameter (MWD) and normalized mean weight diameter (NMWD) of aggregates in the green manure treatment were larger than that with the winter fallow. Soil given green manure retained both a higher water holding capacity in the plow layer soil, and a larger volume of moisture at all matric potentials (-10, -33 and -100 kPa). We conclude that the management of double-rice fields in southern central China should be encouraged to use green manures along with chemical fertilizers to increase SOC content, improve soil physical properties and soil fertility.