[Effects of long-term fertilization on soil organic carbon pool and carbon sequestration under double rice cropping].

This paper studied the effects of 30 years (1981-2010) fertilization with chemical N, P, and K, pig manure (PM), and rice straw (RS) on the soil organic carbon (SOC) and its components contents under intensive double rice cropping. The experiment was established on a typic Hapli-Stagnic Anthrosols in Hunan in 1981, and the soil samples were collected in November 2010. In treatment NPK, the contents of SOC, particulate organic C (POC), and KMnO4-oxidizable C (KMnO4-C) were higher than those in treatments NP and NK. The combined application of chemical and organic fertilizers (treatments NK+PM, NP+RS, and NPK+RS) made the contents of SOC, POC, and KMnO4-C have a significant increase, as compared with chemical fertilizations. Treatment NK+PM had the highest contents of SOC (84.71 t C.hm-2), POC (8.94 t C.hm-2), and KMnO4-C (21.09 t C.hm-2) in top soil (0-45 cm), followed by treatment NPK+RS. Treatment NK+PM had the highest C sequestration (485 kg C.hm-2.a-1) , followed by treatment NPK+RS (375 kg C.hm-2.a-1). The C sequestration efficiency (CSE) of SOC in the treatments of chemical fertilizers plus pig manure or rice straw was obviously higher than that in the treatments of chemical fertilizations, and the CSE of the POC in fertilization treatments (ranging from 0.4% and 1.2%) was lower than that of the KMnO4-C (ranging from 3.0% to 8.3%). By using the values of humification constant (h) and the decay constant (k) in Jenkinson' s equation, it was possible to predict the SOC storages in different treatments in the year 2010; and by using Jenkinson' s equation, it was possible to calculate the C input required to maintain the SOC storages in the year 1981 (AE). The increase of the SOC in treatments NK+PM, NP+RS, and NPK+RS was due to the annual C input being higher than the AE. It was considered that in the double rice cropping areas in subtropical region of China, long-term application of chemical fertilizers combined with pig manure or rice straw could promote the organic carbon sequestration in paddy soils.

[Physical properties of paddy soils with different productivity in double-rice cropping region of Hunan Province].

A total of 24 paddy soil samples with different productivity were collected from eight main rice production counties in east, central, and northwest Hunan Province, with the parameters of soil physical properties and clay mineral composition, including bulk density, particle density, porosity, field water-holding capacity, water-stable aggregates, mean weight diameter, normalized mean weight diameter, aggregate destruction rate, organic carbon, organic cementing materials, inorganic cementing materials, and clay minerals determined. The results showed that highly productive (>14,000 kg x hm(-2)) paddy soil possessed bulk density below 1.2 g x cm3, soil particle density about 2.0 g x cm3, and 5-0.5 mm water-stable macroaggregates about 20%. Most of the test parameters had no significant differences between highly productive and medium productive (10,000-13,500 kg x hm(-2)) paddy soils, but significant differences between highly productive and lowly productive (<10,000 kg x hm(-2)) paddy soils, suggesting that under good management, medium productive paddy soils could be upgraded to highly productive paddy soils. At present, the physical properties of lowly productive paddy soils were poor, which need to be improved to raise productivity.

[Effects of long-term fertilization on reddish paddy soil quality and its evaluation in a typical double-rice cropping region of China].

In order to quantify the effects of 27 years application of chemical fertilizers, pig manure, and rice straw on the reddish paddy soil quality in double rice cropping region, the indices of soil bulk density, porosity, maximum water holding capacity, normalized mean weight diameter, pH, cation exchange capacity, available nutrients, organic matter, microbial biomass C, enzyme activities, and rice yield were selected as the evaluation indicators, and classified into four functional groups, i. e., resistance to physical degradation, plant nutrients supply and storage, resistance to biochemical degradation, and sustaining of crop productivity. The soil quality index (SQI) was calculated based on the four functional groups. The results showed that the SQI ranged from 0.544 in treatment CK to 0.729 in treatment NPK plus rice straw. Treatments PK, NP, and NK induced soil degradation, compared to treatment NPK. The deficiencies of soil P and K were the main limiting factors for the double rice productivity in reddish paddy soil area. Even though 30 t x hm(-2) x a(-1) of pig manure and 4.2 t x hm(-2) x a(-1) of rice straw were applied, the soil P and K were still not adequate for the requirement of rice growth. There was no obvious effect of long-term application of lime on the reddish paddy soil quality. The combined application of NPK with organic manure was an important and effective measure in improving soil quality in double rice cropping regions of Southern China.

[Effects of controlled release nitrogen fertilizer on surface water N dynamics and its runoff loss in double cropping paddy fields in Dongtinghu Lake area].

By using leakage pond to simulate the double cropping paddy fields in Dongtinghu Lake area, this paper studied the effects of urea (CF) and controlled release nitrogen fertilizer (CRNF) on the dynamics of surface water pH, electrical conductivity (EC), total nitrogen (TN), ammonia nitrogen (NH4(+)-N) and nitrate nitrogen (NO3(-)-N) and the runoff loss of TN in alluvial sandy loamy paddy soil and purple calcareous clayed paddy soil, the two main paddy soils in this area. The results showed that after applying urea, the surface water TN and NH4(+)-N concentrations reached the peak at the 1st and 3rd day, respectively, and decreased rapidly then. Surface water NO3(-)-N concentration was very low, though it showed a little raise at the 3rd to 7th day after applying urea in purple calcareous clayed paddy soil. In early rice field, surface water pH rose gradually within 15 days after applying urea, while in late rice field, it did within 3 days. EC kept consistent with the dynamics of NH4(+)-N. CRNF, especially 70% N CRNF, gave rise to distinctly lower surface water pH, EC, and TN and NH4(+)-N concentrations within 15 days after application, but NO3- concentration rose slightly at late growth stages, compared with urea application. The monitoring of TN runoff loss indicated that during double cropping rice growth season, the loss amount of TN under urea application was 7.70 kg x hm(-2), accounting for 2.57% of applied urea-N. The two runoff events occurred within 20 days after urea application contributed significantly to the TN runoff loss. CRNF application resulted in a significantly lower TN concentration in runoff water from the 1st runoff event occurred within 10 days of its application, and thereafter, the total TN runoff loss for CRNF and 70% N CRNF application was decreased by 24.5% and 27.2%, respectively, compared with urea application.