Specific contributions of decaying alfalfa roots to nitrate leaching in a Kalamazoo loam soil

Alfalfa (Medicago sativa L.) contributes 430 million kg N year(-1) to the US Corn-Belt soils, according to a 1991 survey. Minimizing leaching losses from these very large N inputs requires a better understanding of the specific root dynamics that relate to the shoot-borne nitrates which have been reported to develop throughout many soil profiles. The objective of the present study was to determine the impact of decaying alfalfa roots on nitrate inputs to soils and on soil hydraulic conductivity properties which affect nitrate leaching. An experiment was initiated in 1994 and data for this report were taken from research on a Kalamazoo loam soil (fine-loamy, mixed, mesic Typic Hapludalf) at the KBS/LTER (long-term ecological research) site in southwestern Michigan, during the period from 1996 through 1997. Soil extractable nitrate (NO3-N) and ammonium (NH4-N) were monitored to soil depths of 150 cm and soil soluble NO3-N and NH4-N were monitored by suction lysimeters to the depth of 65 cm. Saturated hydraulic conductivity (K-sat) of soil was measured by the double-ring infiltrometer method. Following glyphosate termination of the alfalfa stands, nitrate-N released from mineralized alfalfa roots plus shoots totaled 75 kg ha(-1). Alfalfa roots generated 36 kg ha(-1) and alfalfa shoots generated 39 kg ha(-1) which accumulated in the Ap horizons from April to July in 1997. The presence of decaying alfalfa roots in the profile quadrupled Ksat values as compared to bare fallow soils. Nitrates released from decomposing alfalfa roots combined with root-enhanced hydraulic conductivities dramatically increased NO3-N leaching following the termination of alfalfa stands. NO3-N leaching to deeper horizons approached 83 kg ha(-1) in root treatments and 144 kg ha(-1) in the root plus shoot treatments during the period from April to December, following alfalfa termination. Our data suggest that under temperate climate such as that of Michigan, groundwater contamination by nitrates can be reduced by terminating alfalfa stands immediately before spring-planting of the subsequent row crop, which can absorb the large quantities of N leaking from decomposing shoots and roots of the legume. (c) 2005 Elsevier B.V. All rights reserved

Soil Respiration in Relation to Photosynthesis of Quercus mongolica Trees at Elevated CO2

Knowledge of soil respiration and photosynthesis under elevated CO2 is crucial for exactly understanding and predicting the carbon balance in forest ecosystems in a rapid CO2-enriched world. Quercus mongolica Fischer ex Ledebour seedlings were planted in open-top chambers exposed to elevated CO2 (EC = 500 µmol mol−1) and ambient CO2 (AC = 370 µmol mol−1) from 2005 to 2008. Daily, seasonal and inter-annual variations in soil respiration and photosynthetic assimilation were measured during 2007 and 2008 growing seasons. EC significantly stimulated the daytime soil respiration by 24.5% (322.4 at EC vs. 259.0 mg CO2 m−2 hr−1 at AC) in 2007 and 21.0% (281.2 at EC vs. 232.6 mg CO2 m−2 hr−1 at AC) in 2008, and increased the daytime CO2 assimilation by 28.8% (624.1 at EC vs. 484.6 mg CO2 m−2 hr−1 at AC) across the two growing seasons. The temporal variation in soil respiration was positively correlated with the aboveground photosynthesis, soil temperature, and soil water content at both EC and AC. EC did not affect the temperature sensitivity of soil respiration. The increased daytime soil respiration at EC resulted mainly from the increased aboveground photosynthesis. The present study indicates that increases in CO2 fixation of plants in a CO2-rich world will rapidly return to the atmosphere by increased soil respiration

Contribution of Eucalyptus Harvest Residues and Nitrogen Fertilization to Carbon Stabilization in Ultisols of Southern Bahia

ABSTRACT: Eucalyptus forests in southern Bahia (BA) are planted in soils with a sandy surface layer and humid tropical climate, conditions that lead to soil carbon (C) decomposition. Recent studies have shown that nitrogen (N) may be important for soil C stabilization. The aim of this study was to evaluate the contribution of Eucalyptus harvest residues and nitrogen fertilization to C stabilization in Ultisols of southern BA. The experiment was conducted in Eucalyptus clonal plantations cultivated in two regions of Eunápolis, BA, Brazil, with different clay content: southern region (140 g kg-1 of clay) and western region (310 g kg-1 of clay). Five treatments were evaluated: one control (CTR), without Eucalyptus harvest residues and N fertilization, and four treatments with harvest residues combined with four rates of N fertilization: 0, 25, 50, and 100 kg ha-1. Soil samples were collected from the 0.00-0.10, 0.10-0.20, 0.20-0.40, and 0.40-0.60 m layers at the beginning and the end of the experiment (36 months). The amount of C and N and the C and N isotopic ratio (δ13C and δ15N) of particulate organic matter (POM) and mineral-associated organic matter (MAOM) were determined. In the southern region after 36 months, the C-MAOM stocks in the 0.00-0.10 m layer of the CTR decreased by 33 %. The addition of harvest residue followed by 100 kg ha-1 N increased C-POM and N-POM stocks (0.00-0.10 m) compared to the CTR, and the final N-POM stocks and residue-C recovery in the surface soil layer were positively correlated with the increase in N fertilization rates. In the western region, residue maintenance resulted in increased C-MAOM stocks (0.00-0.10 m) compared to the CTR, but an increase in N availability reduced this increment. The increase in N fertilization rates did not alter C stocks, but reduced N stocks of POM and MAOM in the upper soil layer. At the end of the experiment, N fertilizer recovery (0.00-0.60 m) was similar among the regions evaluated. In soil with lower clay content, higher N availability led to higher C and N stocks in the particulate fraction. In soils with high clay content, physical and chemical protections are more important than N fertilization for soil C stabilization, and just maintaining harvest residues may suffice to increase C and N in the more stable SOM fraction

No depth-dependence of fine root litter decomposition in temperate beech forest soils

Aims Subsoil organic carbon (OC) tends to be older and is presumed to be more stable than topsoil OC, but the reasons for this are not yet resolved. One hypothesis is that decomposition rates decrease with increasing soil depth. We tested whether decomposition rates of beech fine root litter varied with depth for a range of soils using a litterbag experiment in German beech forest plots. Methods In three study regions (Schorfheide-Chorin, Hainich-Dün and Schwäbische-Alb), we buried 432 litterbags containing 0.5 g of standardized beech root material (fine roots with a similar chemical composition collected from 2 year old Fagus sylvatica L. saplings, root diameter<2mm) at three different soil depths (5, 20 and 35 cm). The decomposition rates as well as the changes in the carbon (C) and nitrogen (N) concentrations of the decomposing fine root litter were determined at a 6 months interval during a 2 years field experiment. Results The amount of root litter remaining after 2 years of field incubation differed between the study regions (76 ± 2 % in Schorfheide-Chorin, 85 ± 2 % in Schwäbische-Alb, and 88±2 % in Hainich-Dün) but did not vary with soil depth. Conclusions Our results indicate that the initial fine root decomposition rates are more influenced by regional scale differences in environmental conditions including climate and soil parent material, than by changes in microbial activities with soil depth. Moreover, they suggest that a similar potential to decompose new resources in the form of root litter exists in both surface and deep soils

Leaching losses of nitrate nitrogen and dissolved organic nitrogen from a yearly two crops system, wheat-maize, under monsoon situations

A large amount of nitrogen (N) fertilizers applied to the winter wheat-summer maize double cropping systems in the North China Plain (NCP) contributes largely to N leaching to the groundwater. A series of field experiments were carried out during October 2004 and September 2007 in a lysimeter field to reveal the temporal changes of N leaching losses below 2-m depth from this land system as well as the effects of N fertilizer application rates on N leaching. Four N rates (0, 180, 260, and 360 kg N ha(-1) as urea) were applied in the study area. Seasonal leachate volumes were 87 and 72 mm in the first and second maize season, respectively, and 13 and 4 mm during the winter wheat and maize season in the third rotational year, respectively. The average seasonal flow-weighted NO(3)-N concentrations in leachate for the four N fertilizer application rates ranged from 8.1 to 103.7 mg N l(-1), and seasonal flow-weighted dissolved organic nitrogen (DON) concentrations in leachate varied from 0.8 to 6.0 mg N l(-1). Total amounts of NO(3)-N leaching lost throughout the 3 years were in the range of 14.6 to 177.8 kg ha(-1) for the four N application rates, corresponding to N leaching losses in the range of 4.0-7.6% of the fertilizers applied. DON losses throughout the 3 years were 1.4, 2.1, 3.6, and 6.3 kg N ha(-1) for the four corresponding fertilization rates. The application rate of 180 kg N ha(-1) was recommended based on the balance between reducing N leaching and maintaining crop yields. The results indicated that there is a potential risk of N leaching during the winter wheat season, and over-fertilization of chemical N can result in substantial N leaching losses by high-intensity rainfalls in summer