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Structural Convergence of Maize and Wheat Straw during Two-Year Decomposition under Different Climate Conditions Xiaoyue Wang,†,∥ Bo Sun,*,† Jingdong Mao,‡ Yueyu Sui,§ and Xiaoyan Cao‡ †

State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, No. 71 East Beijing Road, Nanjing 210008, China ‡ Department of Chemistry and Biochemistry, Old Dominion University, 4541 Hampton Boulevard, Norfolk, Virginia 23529, United States § Northeast Institute of Geography and Agricultural Ecology, Chinese Academy of Sciences, Harbin 150040, China ∥ Graduate School of the Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Straw decomposition plays an important role in soil carbon sequestration. Litter quality and climate condition are considered to be key factors that regulate straw decomposition. This study investigated the decomposition characteristics of wheat and maize straw under cold temperate, warm temperate, and midsubtropic climate conditions, and examined whether the chemical structures of straw residues became similar during decomposition under different climate conditions. Straws were put in 0.074-mm-mesh size litter bags to exclude soil fauna and buried in black soil plots at three experimental stations located in the aforementioned climate regions to rule out the impact of soil type. The decomposition rate constants of wheat straw and maize straw increased linearly with temperature, and the former was more sensitive to temperature. Climate conditions and straw quality had marked effects on the residual material structure in the first half year of decomposition, but then decreased. Wheat and maize straw showed common decomposition characteristics with a decrease of O/N-alkyl carbons and di-O-alkyls, and a simultaneous increase of alkyl carbons, aromatic carbons, aromatic C−O groups, and COO/N−CO groups. Overall, the results indicated that the chemical compositions of the two types of straw became similar after 2-year decomposition under different climate conditions.

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INTRODUCTION Straw decomposition converts the products of photosynthesis into stable soil organic matter, which plays an important role in carbon sequestration in farmland ecosystems and mitigates climate change.1 Approximately 34.4 Tg yr−1 of crop residue is produced worldwide, which may lead to sequestration of 200 Tg C yr−1.2 Plant residue decomposition is controlled by, among other factors, the quality of the residue, climate conditions, and soil type.3 Climate conditions are critical factors that control the decomposition of plant residues on a large geographical scale.4,5 Under favorable climatic conditions, residual quality becomes a more important determinant. Additionally, the composition of soil microbes6 and animal communities,7 which changes with soil type, can affect residual decomposition.8,9 Indeed, these factors can sometimes change the regional pattern of residual decomposition along climatic gradients.10,11 During long-term decomposition, climate factors and litter quality only regulate decomposition rates during the early- and midphase, while soil Mn content plays an important role in the late phase.12 To © 2012 American Chemical Society

date, the relative importance of these factors is still not clear at the broad spatial and long temporal scales. It has been proposed that plant materials decompose via similar pathways and produce the same end-products during a lengthy decomposition process which may take several years or even decades to complete.13,14 The chemical structure of different plant materials changes during decomposition, which in turn affects the decomposability of remaining residues. Increasing similarity among chemical structures of different litter residues was reported as decomposition proceeded in a forest ecosystem.15 However, there is currently no evidence supporting the structural similarity of remaining residues during decomposition under different climates. The north−south transect of Eastern China is a longitude transect driven by heat-gradient that covers different soil types. In this study, the black soil from the cold temperate region was Received: Revised: Accepted: Published: 7159

February 8, 2012 June 4, 2012 June 5, 2012 June 5, 2012 dx.doi.org/10.1021/es300522x | Environ. Sci. Technol. 2012, 46, 7159−7165

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Table 1. Weight Loss and Changes of C/N in Residual Straw over Decomposition Perioda mass loss (%) location

0.5 year

1 year

2 years

maize straw

Hailun Fengqiu Yingtan Hailun Fengqiu Yingtan

48.91ef 67.96bcd 66.18bcd 61.74cdef 72.11abcd 74.59abc

46.31f 76.54abc 68.15bcd 55.90def 64.75bcde 70.54bcd

69.99bcd 87.74a 80.14ab 75.47abc 76.62abc 76.64abc

wheat straw

a

C/N

straw

initial 71.51

57.72

0.5 year

1 year

2 years

67.27a 28.09 cd 33.51c 31.70c 18.88e 18.70e

57.96b 22.48de 22.51de 21.85de 15.91e 16.04e

30.64c 16.10e 18.56e 17.91e 18.03e 17.35e

Different letters within a row indicate significant differences between treatments (Tukey’s test, p < 0.05, n = 3).

transferred into warm temperate and midsubtropical regions to conduct a 2-year straw decomposition experiment. The overall goal of this work was to study the impact of climate and straw quality on straw decomposition. The specific objectives were (1) to examine the relative contribution of climate conditions and straw types on straw decomposition by ruling out the influence of soil type, and (2) to determine the chemical and structural changes in residual straw and assess their similarity during decomposition.

layer of soil. Three nylon mesh bags were removed after 0.5, 1, and 2 years, respectively and the residual straw samples were removed from the nylon bags and weighed. After drying, the samples were ground to pass through 0.15 mm (100 mesh) screen for chemical property analysis and ground into powder for NMR analysis. NMR Spectroscopy. 13C NMR analyses were performed using a Bruker Avance III 300 spectrometer at 75 MHz (300 MHz 1H frequency). All experiments were run in a doubleresonance probe head using 4-mm sample rotors. Semiquantitative compositional information was obtained with good sensitivity using the 13C cross-polarization/magic angle spinning (CP/MAS) NMR technique, with a spinning speed of 5 kHz, contact time of 1 ms, and 1H 90° pulse-length of 4 μs. Four-pulse total suppression of sidebands (TOSS)16−18 was employed before detection, with two-pulse phase modulated (TPPM) decoupling applied for optimum resolution. Subspectra for nonprotonated and mobile carbon groups were obtained by combining the 13C CP/TOSS sequence with 40-μs dipolar dephasing (CP/TOSS/DD).16,17 There were 6144 13C CP/TOSS and 13C CP/TOSS/DD scans for all samples. Physical, Chemical, and Biochemical Properties. The decomposition of wheat and maize straw was expressed in percentage of mass loss. The exponential equation Y = Y0 + ae‑Kt was used to simulate the decomposition process, where Y is the weight percentage of initial mass remaining at time t (years), K is the decomposition rate constant calculated by the least-squares method, a is the percentage of initial mass of material subject to loss, and Y0 is the asymptote. Total nitrogen (TN) and total carbon (TC) were determined by Kjeldahl digestion and dichromate oxidation, respectively.19,20 Statistical Analysis. The weight loss data were subjected to ANOVA using the Statistical Package for Social Science (SPSS 13). Significance among means was identified using Turkey’s test at p = 0.05. Principal component analysis (PCA) of the composition of functional groups in residual straw was performed using SPSS 13. Canonical correspondence analysis (CCA)-based variation partitioning analysis (VPA) was performed by R language 2.13.1.

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MATERIALS AND METHODS Experimental Setup and Sampling. The decomposition experiments were set up on May 13, 2008 based on soil transplantation experiments, which were installed in three agroecological experimental stations of the Chinese Ecological Research Network, Hailun (47°26′ N, 126°38′ E), Fenqiu (35°00′ N, 114°24′ E), and Yingtan (28°15′ N, 116°55′ E), in October 2005. These selected stations represent a heat gradient from cold temperate (Hailun) to warm temperate (Fengqiu) to midsubtropical regions (Yingtan) (Supporting Information). The average annual temperature at the aforementioned stations is 1.5, 13.9, and 17.6 °C, respectively. The black soil (Phaeozem) from Hailun station was selected. This soil has a pH of 6.3 and a SOM content of 48.7 g kg−1. The soil was transferred to the other two experimental stations and immediately placed into the experimental plots layer by layer (20 cm per layer) according to the original sequence of soil layers. Black soil was selected to set up the transplantation experiments (1) to identify the relative contribution of climate on straw decomposition in the same type of soil, (2) to discuss the effect of increasing air temperature on soil organic carbon turnover by building a temperature gradient for the black soil from Hailun in the cold temperate region to Yingtan in the midsubtropic region. The basic chemical properties of the soil before the decomposition experiment (April 2008) were determined and are shown in Table S1. The experiment plot was 1.2 m wide ×1.4 m long ×1 m deep, and surrounded by a 20-cm-thick cement cell wall (30 cm above ground). The internal wall was covered by waterproof membranes to prevent the infiltration of water and fertilizers. The bottom of the experimental plot was paved with 5 cm of quartz sand. Maize straw and wheat straw were placed in sample bags made of double-layer nylon mesh that were 15 cm long and 10 cm wide. The mesh size of the litter bags was 0.074 mm, which would prevent soil particles from mixing with the straw treatments or their residues while allowing access by soil water and microorganisms. Each bag contained 100 g of maize or wheat straw cut into 5 cm pieces. The bags were inserted vertically into the soil profile, which was covered with a 5-cm

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RESULTS Decomposition Patterns and Changes in Chemical Properties of Straw. Table 1 shows the decomposition pattern of wheat and maize straw. Generally, wheat straw decomposed faster than maize straw, and straw at Fengqiu and Yingtan stations decomposed more rapidly than that at Hailun station. After 2 years of decomposition, maize and wheat straw added to the same soil under different climate conditions had decomposed to similar amounts (mass loss of approximately 7160

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years of decomposition. The results of multifactor analysis of variance (Table S4) showed that both straw weight loss and C/ N ratio were significantly (p < 0.001) affected by the interaction of all factors. NMR Spectroscopy. The 13C NMR spectra of fresh and decomposed maize straw samples are shown in Figure 2 and

80%), except for maize straw at Hailun station (ca. 70%) and Fengqiu station (ca. 88%). The decomposition rate constants (K) of maize straw were 2.09 y−1 (Hailun), 2.99 y−1 (Fengqiu), and 3.80 y−1 (Yingtan), while those for wheat straw were 3.76 y−1 (Hailun), 6.76 y−1 (Fengqiu), and 8.51 y−1 (Yingtan) (Table 2). In addition, the decomposition rate constant (K) increased with temperature for both wheat straw and maize straw, and the former increased more rapidly (Figure 1). Table 2. Regression Models for the Mass of Remaining Maize and Wheat Straw after Two Years of Decomposition under Three Climate Conditions straw maize straw wheat straw

location Hailun Fengqiu Yingtan Hailun Fengqiu Yingtan

regression models y y y y y y

= = = = = =

33.51 14.79 24.18 29.60 25.52 24.36

+ + + + + +

−2.09t

66.49e 85.21e−2.99t 75.82e−3.80t 70.40e−3.76t 74.48e−6.76t 75.64e−8.51t

R2 0.95 0.99 0.98 0.98 1.00 1.00

Figure 2. CP/TOSS spectra (thin lines) and CP/TOSS/DD spectra (thick lines) of original and decomposed maize straw.

those of fresh and decomposed wheat straw samples are presented in Figure 3. The spectra of maize and wheat straw

Figure 1. Simulation of decomposition by exponential equations showed that the decomposition rate constant (K) increased linearly with mean annual temperature during 2 years of decomposition.

For both wheat and maize straw, most of the mass was lost during the first half year. Moreover, the half-year mass loss for both types of straw was lowest at Hailun station, while the loss at this station became the greatest among sites after 2 years. The half-year mass loss of wheat straw was larger than that of maize straw, but the mass loss of maize straw from year 1 to year 2 was approximately twice that of wheat straw at Hailun and Yingtan station. The fresh maize straw had a significantly higher C/N ratio than the wheat straw (Table 1). As decomposition proceeded, the C/N ratio decreased for both types of straw. The residual maize straw generally showed a higher C/N ratio than wheat straw during decomposition. After 2 years of decomposition, the residual straw at all stations had a similar C/N ratio of approximately 17, except for maize straw at Hailun station (ca. 30). Climate conditions also exerted an impact on maize straw decomposition over 2 years. The residual maize straw with the same decomposition time had a significantly higher C/N ratio at Hailun station than at Fengqiu and Yingtan stations. This trend was less obvious for wheat straw. The C/N ratio of residual wheat and maize straw (except for residual maize straw at Hailun station) was similar among the three sites after 2

Figure 3. CP/TOSS spectra (thin lines) and CP/TOSS/DD spectra (thick lines) of original and decomposed wheat straw.

residues after 1 year of decomposition are not shown because they were very similar to those of the 0.5 year residues. 13C CP/ TOSS spectra (thin lines) showed signals from all carbon sites, while CP/TOSS with dipolar dephasing (CP/TOSS/DD) spectra (thick lines) highlighted signals from nonprotonated carbons and mobile carbon groups. The CP/TOSS spectra of fresh maize and wheat straw samples were quite similar and both exhibited strong and sharp signals from oxygenated aliphatic carbons in carbohydrates. The CP/TOSS/DD spectra of original wheat and maize straw retained signals from CH3, OCH3, nonprotonated di-O-alkyl, nonprotonated CC, aromatic C−O, and COO/N−CO carbons. The CP/ TOSS and CP/TOSS/DD spectra confirmed the contributions to the peak at ∼57 ppm from both OCH3 and NCH. The resonance at ∼20 ppm (CH3) and ∼174 ppm (COO/N−C O) indicated the presence of CH3COO in hemicellulose. The signals at ∼57 ppm (NCH) and ∼174 ppm (COO/N−CO) were partially associated with proteins or peptides. Signals from OCH3 and aromatic C−O carbons indicated the presence of 7161

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of maize straw at Yingtan and Fengqiu stations appeared to increase with time. Wheat straw was shown to be more easily decomposed than maize straw. Decomposition of wheat straw started during the first half year at Hailun station, as indicated by changes in functional groups similar to those observed for maize straw at Fengqiu station. The relative abundances of functional groups remained rather constant in residual wheat straw after 1 year of decomposition when compared to those after 0.5 year of decomposition (Figure 4). However, further decomposition occurred in residual wheat straw after 2 years, with change tendencies similar to those observed in the first half year of decomposition but with less intensity occurring. After the first half year, decomposition of wheat at Fengqiu and Yingtan stations was less obvious, as indicated by only slight changes in its composition and mass loss occurring. Residual wheat straw again underwent the largest decomposition at Fengqiu station, followed by that at Yingtan station and then Hailun station. Different from maize straw, the decomposition of wheat straw did not increase much after half a year. Researchers found that O-alkyl C decreased, but alkyl C and aromatic C increased during decomposition.21,22 The ratio of O-alkyl/alkyl and aromaticity can be used as indexes of decomposition dynamics. The O-alkyl/alkyl ratio (Figure 5) of

lignin. The signals in the unsaturated carbon region between 110 and 140 ppm were attributed to aromatic carbons in lignin and/or olefinic carbons in lipids. Overall, these results indicated the predominance of cellulose and/or hemicellulose in original wheat and maize straw samples, as well as the presence of lipids, proteins, and lignin in much less abundance. The changes in the relative abundance of functional groups derived from 13C NMR spectra in maize and wheat straw residues with decomposition are shown in Figure 4. Residual

Figure 4. Composition of functional groups (%) in maize and wheat straw samples obtained by the CP/TOSS technique. HL = Hailun, FQ = Fengqiu, YT = Yingtan. The content of O/N-alkyl C (%) is shown by the secondary axis.

Figure 5. O-alkyl/alkyl ratio and aromaticity under three climate conditions during decomposition. Aromaticity = aromatics/(alkyl + Oalkyl + aromatic)5 (alkyl refers to the spectral region at 0−45 ppm, Oalkyl refers to the region at 45−90 ppm, and aromatic refers to the region at 110−1606,7). HL = Hailun, FQ = Fengqiu, YT = Yingtan. The value of aromaticity is shown by secondary axis.

maize straw materials at Hailun station showed rather small differences in distribution of functional groups during 2 years of decomposition (Figure 4), suggesting a low degree of decomposition. After 0.5 year of decomposition, residual maize straw at Fengqiu station showed a decrease of O/Nalkyl and di-O-alkyl, accompanied by an increase of all other functional groups. After 2 years of decomposition, residual maize straw displayed the lowest contents of O/N-alkyl and diO-alkyl, but highest contents of alkyl carbons, aromatic C, aromatic C−O groups, and carbonyl groups, indicating the highest degree of decomposition. As the decomposition time increased, residual maize straw at Yingtan station exhibited a progressive decrease of O/N-alkyl and di-O-alkyl, but a gradual increase in all other functional groups (alkyl carbons, aromatic C, aromatic C−O, and carbonyl). The extent of decomposition

maize straw was much larger than that of wheat straw. This ratio of residual maize straw samples was generally highest at Hailun station, followed by Fengqiu and then Yingtan station. For wheat straw, this ratio was largest at Hailun station, but lowest at Fengqiu station. The O-alkyl/alkyl ratio decreased with decomposition, while aromaticity generally showed the opposite trend. To separate and evaluate the effects of climate, straw type, and decomposition time on decomposition, CCA-based variation partitioning analysis was performed.23 The C/N and O-alkyl/alkyl ratios and aromaticity (Supporting Information) were considered as straw type variables. The average temperature and precipitation during decomposition were considered to be climate variables. Climate, straw type, and decomposition time made the greatest contribution to decomposition (Figure 6). Among these three factors, straw type was dominant. 7162

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than maize straw. In the present study, the decomposition rate constants (K) for wheat straw were higher than those of maize straw (low quality) (Table 2). In addition, the temperature sensitivity of organic matter decomposition can vary depending on straw type. Specifically, recalcitrant material is expected to be more sensitive to temperature than labile fractions because decomposition of recalcitrant material requires a higher activation energy.29−31 However, wheat straw with higher quality showed a higher temperature sensitivity during the 2 years of decomposition, as indicated by its rate constant (K) increasing faster with temperature when compared to that of maize straw (Figure 1). This could be explained by the fact that more recalcitrant carbons were accumulated in wheat straw than maize straw after a half year of rapid decomposition, as indicated by the NMR data. Temperature and water availability have been shown to be the major climate factors controlling litter decomposition on large spatial scales,32 while on small scales litter quality can be the key factor.33,34 However, in the present study (Figures 6 and 7), we found that litter quality was the major factor controlling decomposition, even on a relatively large spatial scale involving three different climate zones. This could be due to difficulty isolating the effects of climate from those of soil type9 and vegetation cover.35 Additionally, in a study conducted by Liski, the litter bag placed on the surface had a mesh size of about 0.6 mm, which was larger than that of the litter bags used in our study (0.074 mm), and allowed soil mesofauna to enter the bags.32 Soil animals have a significant effect on litter decomposition and the relative contribution of soil fauna to decomposition was dependent on climate.36 By excluding the influence of soil type, vegetation cover, and soil microorganisms, litter quality became the major controlling factor in our study. The main structural changes during decomposition were a decrease in di-O-alkyl C and O/N-alkyl C and an increase of alkyl carbons, aromatic carbons, aromatic C−O groups, and COO/N−CO groups. These changes were indicative of the loss of proteinaceous and carbohydrate materials and the accumulation of recalcitrant materials containing a high proportion of aromatic carbons during decomposition.37,38 It has been suggested that organic materials are degraded via similar pathways and thus lead to similar residues, despite different litter chemistry and environmental conditions.39 The variation of C/N between the two types of straw under three climate conditions decreased with decomposition, and the coefficient of variation (CV) between treatment means was calculated to be 54.2%, 60.9%, and 27.3% for year 0.5, 1, and 2, respectively. The NMR data also revealed decreasing variations in the distributions of functional groups (Figure 6). The CV of the O-alkyl/alkyl ratio was 57.3%, 40.2%, and 34.4% for year 0.5, 1, and 2, respectively, which confirms the similarity-trend hypothesis presented above. Warmer and drier climatic conditions have been found to lead to faster decomposition and hence accumulation of more recalcitrant materials.40,41 Because more recalcitrant materials accumulated in straw residues at Yingtan station than at Fengqiu and Hailun, the straw residues at Yingtan decomposed much more slowly during later stages; thus, their mass loss was smaller and their chemical composition changed slowly.26 Accordingly, the mass remaining and chemical structures became similar among the three geographic sites after 2 years of decomposition. In addition, climate regulates decomposition by affecting soil microorganisms. However, the decomposition

Figure 6. CCA-based variation partitioning analysis (VPA) shows the relative contribution of different types of factors to straw decomposition (mass remaining). C = climate condition, S = straw quality, T = decomposition time.

Principal component analysis (PCA) based on the relative abundances of functional groups (Figure 7) showed that the

Figure 7. Principal component analysis of the composition of functional groups. HL = Hailun, FQ = Fengqiu, YT = Yingtan, WS = wheat straw, MS = maize straw, 0 = 0 year, 0.5 = half year decomposition, 1 = 1-year decomposition, 2 = 2-year decomposition. Figures beside the marks represent the decomposition time.

points reflecting maize straw and wheat straw samples were significantly separated. These findings indicate that straw type was the main factor determining the distributions of different functional groups. After 2 years of decomposition, the sample points approached the first quadrants (the circle in Figure 7), except for maize straw at Hailun station, which was indicative of the similar compositions of functional groups of residual straws at the other two stations and wheat straw at Hailun station. The chemical composition of wheat straw showed less change over time, while maize straw showed significant changes, except at Hailun station due to the low temperature.

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DISCUSSION The decomposition of straw observed in the present study was similar to that of other studies of straw decomposition,24 as indicated by similar C/N ratios (28−35 in a study by Christensen24 vs 16−30 in the present study) and mass loss after 2 years of decomposition (67−92% previously24 vs 70− 88% in this study). The straw decomposed slightly more rapidly than litter in forest systems.10,25 Low-quality plant materials with high C/N ratios and high levels of lignin and aromatic compounds decompose more slowly than high-quality plant materials.26,27 The mean contents of lignin and hemicellulose are 11.7% and 30.7% in wheat straw and 19.3% and 21.2% in maize straw, respectively.28 Accordingly, wheat straw is of higher quality 7163

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(9) Hilli, S.; Stark, S.; Derome, J. Litter decomposition rates in relation to litter stocks in boreal coniferous forests along climatic and soil fertility gradients. Appl. Soil Ecol. 2010, 46, 200−208. (10) Gholz, H. L.; Wedin, D. A.; Smitherman, S. M.; Harmon, M. E.; Parton, W. J. Long-term dynamics of pine and hardwood litter in contrasting environments: Toward a global model of decomposition. Global Change Biol. 2000, 6, 751−765. (11) Parton, W.; Silver, W. L.; Burke, I. C.; Grassens, L.; Harmon, M. E.; Currie, W. S.; King, J. Y.; Adair, E. C.; Brandt, L. A.; Hart, S. C. Global-scale similarities in nitrogen release patterns during long-term decomposition. Science 2007, 315, 361−364. (12) Berg, B.; Johansson, M. B.; Meentemeyer, V. Litter decomposition in a transect of Norway spruce forests: Substrate quality and climate control. Can. J. For. Res. 2000, 30, 1136−1147. (13) Mathers, N. J.; Jalota, R. K.; Dalal, R. C.; Boyd, S. E. 13C-NMR analysis of decomposing litter and fine roots in the semi-arid Mulga Lands of southern Queensland. Soil Biol. Biochem. 2007, 39, 993− 1006. (14) Sjogersten, S.; Turner, B. L.; Mahieu, N.; Condron, L. M.; Wookey, P. A. Soil organic matter biochemistry and potential susceptibility to climatic change across the forest-tundra ecotone in the Fennoscandian mountains. Global Change Biol. 2003, 9, 759−772. (15) Preston, C. M.; Nault, J. R.; Trofymow, J. Chemical changes during 6 years of decomposition of 11 litters in some Canadian forest sites. Part 2. 13 C abundance, solid-state 13 C NMR spectroscopy and the meaning of lignin. Ecosystems 2009, 12, 1078−1102. (16) Mao, J.; Chen, N.; Cao, X. Characterization of humic substances by advanced solid state NMR spectroscopy: Demonstration of a systematic approach. Org. Geochem. 2011, 42, e169−e190. (17) Cao, X.; Olk, D. C.; Chappell, M.; Cambardella, C. A.; Miller, L. F.; Mao, J. Solid-State NMR Analysis of Soil Organic Matter Fractions from Integrated Physical−Chemical Extraction. Soil Sci. Soc. Am. J. 2011, 75, 1374−1384. (18) Dixon, W. T. Spinning-sideband-free and spinning-sidebandonly NMR spectra in spinning samples. J. Chem. Phys. 1982, 77, 1800. (19) Novozamsky, I.; Houba, V.; Van Eck, R.; Van Vark, W. A novel digestion technique for multi-element plant analysis. Commun. Soil Sci. Plant Anal. 1983, 14, 239−248. (20) Nelson, D. W.; Sommers, L. E.; Sparks, D.; Page, A.; Helmke, P.; Loeppert, R.; Soltanpour, P.; Tabatabai, M.; Johnston, C.; Sumner, M. Total carbon, organic carbon, and organic matter. In Methods of Soil Analysis. Part 3-Chemical Methods; Bigham, J. M., Ed.; American Society of Agronomy: Madison, WI, 1996; pp 961−1010. (21) Baldock, J. A.; Oades, J. M.; Nelson, P. N.; Skene, T. M.; Golchin, A.; Clarke, P. Assessing the extent of decomposition of natural organic materials using solid-state 13C NMR spectroscopy. Aust. J. Soil Res. 1997, 35, 1061−1084. (22) Golchin, A.; Baldock, J.; Oades, J., A model linking organic matter decomposition, chemistry, and aggregate dynamics. In Soil Processes and The Carbon Cycle; Lal, R., Kimble, J., Follett, R., Stewart, B., Eds.; CRC Press: Boca Raton, FL, 1997; pp 245−266. (23) Borcard, D.; Legendre, P.; Drapeau, P. Partialling out the spatial component of ecological variation. Ecology 1992, 73, 1045−1055. (24) Christensen, B. T. Wheat and barley straw decomposition under field conditions: Effect of soil type and plant cover on weight loss, nitrogen and potassium content. Soil Biol. Biochem. 1985, 17, 691− 697. (25) Chen, H.; Harmon, M. E.; Sexton, J.; Fasth, B. Fine-root decomposition and N dynamics in coniferous forests of the Pacific Northwest, USA. Can. J. For. Res. 2002, 32, 320−331. (26) Carvalho, A.; Bustamante, M.; Alcantara, F.; Resck, I.; Lemos, S. ̅ Characterization by solid-state CPMAS 13C NMR spectroscopy of decomposing plant residues in conventional and no-tillage systems in Central Brazil. Soil Tillage Res. 2009, 102, 144−150. (27) Preston, C.; Bhatti, J.; Flanagan, L.; Norris, C. Stocks, chemistry, and sensitivity to climate change of dead organic matter along the Canadian boreal forest transect case study. Clim. Change 2006, 74, 223−251.

of soil recalcitrant organic matter has been shown to be unrelated to the microbial community.42 Thus, the impact of climate on recalcitrant materials was weakened in the later decomposition stage. Similarly, since recalcitrant materials accumulated in maize and wheat straw residue, the relative effects of the initial quality decreased. These factors could explain the weakened effect of climate and litter chemistry during the latter stage and the convergence of chemical composition. Overall, the straw quality had a greater effect on the structure of residual material during the first half year of decomposition than climate conditions; however, the difference between the impact of these factors decreased and the organic compositions of the two types of straws became similar after 2 years of decomposition under different climate conditions. Further research should be conducted to examine the relative contribution of straw quality in other types of soil and the relationship between the change of residual straw structure and the evolution of microbial communities.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experiment setup and data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: 86-25-86881282; fax: 86-2586881000. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Juanmin Rong and Bin Kong for help in sampling and chemical analysis. This work was supported by the National Basic Research Program of China (2011CB100506), National Natural Science Foundation of China (40871123), and Knowledge Innovation Program of Chinese Academy of Sciences (KZCX2-YW-407).

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REFERENCES

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dx.doi.org/10.1021/es300522x | Environ. Sci. Technol. 2012, 46, 7159−7165