Mingkui Zhang

Long-term changes in organic carbon and nutrients of an Ultisol under rice cropping in southeast China

It is well known that the availability of nutrients in red soils (equivalent to Ultisols and some of the Alfisols and Oxisols in the soil taxonomy of USA) changes after conversion of upland to irrigated rice (Oryza sativa L.) production, but long-term changes in carbon (C) and nutrients are not well documented. To characterize changes in C and nutrients in paddy fields on a Quaternary red clay (clayey, kaolinitic thermic typic plinthudults) during long-term rice cropping, we measured total C, nitrogen (N), phosphorus (P) and potassium (K), particulate organic matter (POM), N in the POM, potential mineralized N, available P, as well as other properties (pH, exchangeable cations, effective cation exchange capacity (ECEC), aggregate stability) in the plow layer (0–15 cm) of 66 rice fields with rice-cultivation time ranging from 2 to 100 years. Total C, N, and P distributions were also determined in six soil profiles with rice-cultivation times of 2, 5, 19, 48, 68, and 100 years, respectively. Significant increases in organic C, total N, and P concentrations in plow layer were found in the first 30–40 years of rice cropping, accompanied by increases in available P and potential mineralized N, exchangeable Ca, Mg, Na, base saturation, and waterstable aggregates, and decreases in total K and clay content. The C/N ratio of organic matter tended to decrease in the first 20 years of rice cropping, and remained constant at approximately 10, whereas the ratio of humic acid to fulvic acid (H/F ratio) increased gradually to about 1 after 50 years of rice cropping. Long-term rice cropping elevated C, N, and P in the plow layer and increased accumulation of C, N, and P in the subsurface soils. The results indicate: (i) long-term rice cropping improved soil fertility as evidenced by neutralization of soil acidity, and increases in ECEC, organic C content, and H/F ratio; (ii) imbalance of fertilization by high N and low K, as revealed by decreased soil K and increased soil N; (iii) long-term rice cropping caused downward movement of organic C, N, and P, which may result in environmental impacts. D 2003 Elsevier B.V. All rights reserved.

Potential Risks of Copper, Zinc, and Cadmium Pollution due to Pig Manure Application in a Soil–Rice System under Intensive Farming: A Case Study of Nanhu, China

Heavy metal (copper [Cu], zinc [Zn], and cadmium [Cd]) pollution of soils from pig manures in soil-rice ( L.) systems under intensive farming was investigated, taking Nanhu, China, as the case study area. Two hundred pig manures and 154 rice straws, brown rice samples, and corresponding surface soil (0-15 cm) samples were collected in paddy fields from 150 farms in 16 major villages within the study area. The mean Cu and Zn concentrations in pig manures consistently exceeded the related standard. About 44 and 60% of soil samples exceed the Chinese Soil Cu and Cd Environmental Quality Standards, respectively. The concentration of Cu, Zn, and Cd in brown rice did not exceed the Chinese Food Hygiene Standard. There was a significant positive correlation between total Cu and Zn contents in soil and application rate of pig manures. Strong correlation was observed between the extractable Cu, Zn, and Cd in soil and the Cu, Zn, and Cd contents in the brown rice. The spatial distribution maps of Cu and Zn concentrations in brown rice, straw, and extractable soil Cu and Zn concentration also showed similar geographical trends. Further analyses on heavy metals loading flux and accumulation rates from pig manure applied suggested that Cu and Cd contents in soil currently have already exceeded the maximum permissible limit, and Zn, if still at current manure application rates, will reach the ceiling concentration limits in 9 yr. This study assists in understanding the risk of heavy metals accumulating from pig manure applications to agricultural soils.

Extractability and Mobility of Copper and Zinc Accumulated in Sandy Soils

ABSTRACT Extractability and mobility of Cu and Zn and their relationships with 1) accumulation of Cu and Zn and 2) soil pH were studied in three sandy soils (Wabasso, Ankona, and Winder) from commercial citrus groves in Florida, USA. The soils, with a broad range of Cu and Zn concentrations, were fractionated by a modified procedure of Amacher, while Cu and Zn mobility were evaluated using column leaching. The extractability of Cu and Zn increased with decreasing soil pH. Also with increasing total soil Cu and Zn for extractable Cu in the Wabasso sand a threshold level, where the metal extraction rate increased, was noted at 100 mg kg−1 whereas for extractable Zn in the Wabasso sand the threshold level was found at 60 mg kg−1 and in the Ankona sand at 120 mg kg−1. These results suggested that the release potential of Cu and Zn was greater in the Wabasso sand than in the Ankona sand. The column leaching experiment showed that at total soil Cu or Zn concentrations 200 mg kg−1 for Cu and > 150 mg kg−1 for Zn with decreasing soil pH, the concentrations of both Cu and Zn in the leachates increased exponentially. Also in these sandy soils soluble Cu and Zn mainly originated from the exchangeable fractions, and pH was a key factor controlling Cu and Zn extractability and mobility.

RELEASE POTENTIAL OF PHOSPHORUS IN FLORIDA SANDY SOILS IN RELATION TO PHOSPHORUS FRACTIONS AND ADSORPTION CAPACITY

ABSTRACT Information on P release potential in relation to labile P and P fractions in sandy soils is limited. In this study, P release potential was determined by leaching, and labile P, soil P fractionation, and P adsorption capacity were measured in the laboratory using 96 Florida sandy soil samples to evaluate the relationship between P release in water and soil P status. The sandy soils had a very low P adsorption capacity. The adsorption maximum, as calculated from the Langmuir equation, averaged 40.4 mg P kg−1. More than 10% of the soil P was water soluble, indicating a high risk of P leaching from soil to water. Successive leaching using deionized water released, on average, 7.7% of total P (144.5 mg kg−1) in different soils, whereas labile P recovered by successive water extraction accounted for 39.2% of the total P. Variation in P release potential among the different soils could be explained more by the difference in amounts of extractable P than the adsorption capacity. Total amounts of P released by successive leaching were significantly correlated with all labile P indices measured by different methods and all soil P fractions except for residual P. The correlation coefficients (r) were 0.97** for water-soluble P, 0.96** for 0.01 M CaCl2-P, 0.94** for Olsen P, 0.86** for Mehlich 1-P, 0.77*** for Mehlich 3-P, and 0.64*** for Bray 1-P. There were no obvious turning points in the relationships between Olsen-P, water-soluble P, or CaCl2-P and the amounts of P released from the sandy soils. The release of P from the sandy soils appeared to be controlled by a precipitation–dissolution reaction rather than a P sorption–desorption process. Furthermore, the sequential extraction of soils using deionized water indicated that P released was not limited to the labile P (H2O-P, NaHCO3-IP) and potentially labile P (NaOH-P) pools, but also from the HCl-P, indicating that all of P fractions except for residual P in the sandy soils can contribute to P release.

Effects of Soil Properties on Phosphorus Subsurface Migration in Sandy Soils

Abstract The soil factors influencing the potential migration of dissolved and particulate phosphorus (P) from structurally-weak sandy subsoils were evaluated by means of soil column leaching experiments. Soil colloids were extracted from two types of soils to make the colloid-bound forms of P solution. Eight sandy soils with diverse properties were collected for packing soil columns. The effects of influent solutions varying in concentrations of colloids, P, and electrolyte, on the transport of P and quality of leachates were characterized. P migration in the soils was soil property-dependent. High soil electrical conductivity values retarded the mobility of colloids and transportability of colloid-associated P (particulate P). Soil electrical conductivity was negatively correlated with colloids and reactive particulate P (RPP) concentrations in the leachates, whereas, the total reactive P (TRP) and dissolved reactive P (DRP) concentrations in the leachates were mainly controlled by the P adsorption capacity and the P levels in the subsoil. The reactive particulate P in the leachates was positively correlated with the colloidal concentration. Increased colloidal concentration in the influent could significantly increase the colloidal concentration in the leachates. Elevated P concentration in the influent had little effect on P recovery in the leachates, but it resulted in significant increases in the absolute P concentration in the leachates.

Leaching Potential of Heavy Metals, Nitrogen, and Phosphate from Compost-Amended Media

Leaching potential of nutrients and heavy metals was evaluated from a peat-based medium (containing 70% peat, 20% perlite, and 10% vermiculite) amended with varying proportions (0%, 25%, 50%, 75%, or 100%) of compost (biosolids and yard waste, 1:1 by weight). The compost contained small amounts of Zn, Cu, Pb, and Cd. However, the leachate fractions of Zn, Cu, Pb, and Cd in compost accounted for only 0.19%, 0.23%, 0.05%, and 0.27%, respectively of the total concentrations. Except for Cu, the concentrations of Zn, Pb, and Cd were higher in the leachates of peat-based medium than the compost amended media. The concentrations of Cd and Pb in the first leachate of the peat-based medium exceeded the drinking water standards (USEPA 1989). However, the concentrations of Cd, Cr, Cu, and Pb in all the compost amended media were below the limit of the drinking water standards. The concentrations of total P and PO4-P in leachates increased with increasing proportion of compost in the media. Concentration of NO3-N in the first leachate was high and decreased in the subsequent leachings for all the compost amended media. These results suggested that the biosolids-yard waste compost may be a safe and acceptable replacement or partial replacement to peat-based medium without increased leachability of nutrients and heavy metals.

Accumulation and partitioning of phosphorus and heavy metals in a sandy soil under long-term vegetable crop production

Abstract Increased inputs of phosphorus (P) and heavy metals to agricultural soils have caused considerable concern. Information on accumulation and chemical forms of the elements in soils is needed as a guide for the judicious application of agricultural chemicals and organic manures. The focus of this study was to assess accumulation of P and heavy metals among various fractions of a sandy soil with a 25 year history of vegetable crop production and primarily inorganic fertilization. The results demonstrated that long-term vegetable production practices changed concentrations and partitioning of P and heavy metals in the soil. Phosphorus, Cu, Zn, and Mn were significantly accumulated and moved downward along the soil profile. Most of the total Cr in the vegetable soil accumulated in the upper 0–15 cm. However, there was no significant accumulation and transport of Cd, Co, Mo, Ni, and Pb in the vegetable soil. Major P fractions in the vegetable soil were NaHCO3-P, followed by HCl-P and residual P. Copper, Zn, and Mn accumulated predominantly in the CaCO3 fraction or oxide fraction, whereas Cr accumulated mainly in the organically bound fraction, indicating that P, Cu, Zn, and Mn in the vegetable soil have greater mobility potential. Compared with adjacent forest soil, the vegetable soil had a lower percentage of P, Cu, Zn, and Mn in the residual fractions, and a higher percentage of P, Cu, Zn, and Mn in the CaCO3 fractions or organically bound fraction.