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Organic C and N mineralization as affected by dissolved organic matter in paddy soils of subtropical China
Geoderma 157 (2010) 206–213
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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a
Organic C and N mineralization as affected by dissolved organic matter in paddy soils of subtropical China Z.P. Li a,b,⁎, C.W. Han a,b, F.X. Han c a b c
State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China Graduate University of the Chinese Academy of Sciences, Beijing 100049, China Institute for Clean Energy Technology, Mississippi State University, 205 Research Blvd, Starkville, MS 39759, USA
a r t i c l e
i n f o
Article history: Received 6 August 2009 Received in revised form 10 April 2010 Accepted 20 April 2010 Available online 20 May 2010 Keywords: DOM dynamics DOM removal Mineralization of C N Incubation Paddy soils Subtropical China
a b s t r a c t In this study, dynamics of dissolved organic matter (DOM) and mineralization of soil organic C and N in subtropical China were investigated. Paddy and upland soils were derived from Tertiary sandstone, Quaternary red clay and river alluvium with low, middle, and high levels of soil fertility. Dissolved organic carbon contents (DOC) varied from 3.8 to 68.7 mg/kg and dissolved organic nitrogen contents (DON) ranged from 2.9 to 18.3 mg/kg, respectively, during the experimental period. Both DOC and DON increased with increasing soil fertility. The content of DOM was characterized by strong seasonal fluctuation in soils, and seasonal patterns showed the maximum value in Nov and the minimum in July. The contribution of DOM to mineralization of C and N was significantly affected by soil types and seasons. DOM removal significantly decreased the cumulative organic C mineralization in soils derived from Tertiary sandstone and River alluvium (by 1.6%–20.3% with an average of 7.1%), while it did not significantly change those in soils derived from Quaternary red clay. Similarly, the cumulative mineralization of N in paddy soils after DOM removal decreased by 6.7%–27.3% over seasons. In this study, DOM played an important role in soil C and N mineralization in subtropical China. It appears that contribution of DOM to minerilization of C and N in paddy soils was possibly related to DOM content and composition. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Mineralization of soil organic matter (SOM) is an important biochemical process related closely to release and supply of plant nutrients, formation and emission of greenhouse gases, and maintenance of soil quality. A better understanding of the dynamics and mechanisms of SOM mineralization is essential so that best management practices are adopted to scientifically manage soil nutrients and mitigate global warming. SOM mineralization is a microbial process and affected by temperature, moisture, soil physical and chemical properties, and agricultural practices (Parton et al., 1987; Wadman and de Haan, 1997; Raiesi, 2006). In addition, chemical composition and nature of SOM is a critical factor for decomposition (Levi-Minzi et al., 1990; Nelson et al., 1994). SOM consists of different biologically-available compounds or fractions. Solid phase is expected to be less susceptible to decomposition while DOM is easily decomposable (Ellert and Gregorich, 1995). Generally, depolymerization and solubilization of SOM were considered to be a prerequisite to mineralization ⁎ Corresponding author. State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China. Tel.: + 86 25 86881505; fax: + 86 25 86881000. E-mail address: [email protected] (Z.P. Li). 0016-7061/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2010.04.015
(Marschner and Kalbitz, 2003). These processes release nutrients to soil solution, which are available to the soil microbial biomass and plants. The soil solution is a bottleneck in the mineralization convertion of solid-phase organic matter to CO2 and CH4 (Marschner and Kalbitz, 2003; Qiu et al., 2008). Therefore, dynamics of DOM play an important role in mineralization of SOM (Gregorich et al., 1998; Motavalli et al., 2000). Numerous studies have examined the pool size and chemical composition of DOM (Christ and David, 1996; Michalzik and Matzner, 1999; Kalbitz et al., 2000; Embacher et al., 2007), while the role of DOM in the microbial mediated mineralization of SOM remains unclear. Burford and Bremner (1975) and Davidson et al. (1987) found a strong correlation between DOC concentration and soil organic carbon (SOC) mineralization in a variety of soil types. However, Cook and Allan (1992) reported that there was no obvious relationship between DOC and instantaneous rates of SOC mineralization over a long incubation period. Liang et al. (1996) suggested that application of high amounts of soluble C (e.g., extracts of composted manure) could accelerate indigenous soil C decomposition. Cookson and Murphy (2004) quantified the importance of DOM in soil C and N cycling. They showed that 2 weeks after DOM removal microbial respiration from soils was not altered, but significant declines in microbial biomass N, potentially mineralizable N, gross N mineralization and gross nitrification occurred. They concluded up to 25% of microbial N supply from DON. Jones and Kielland (2002)
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and Jones et al. (2004) proposed that the conversion of insoluble organic + − N to DON, rather than DON to NH+ 4 or NH4 to NO3 actually was a major constraint to N supply. About 93% of paddy field in China is distributed in the tropical and subtropical regions (Li, 1992). Rice cultivation areas have steadily increased in subtropical China in the past decades (Du and He, 2001). The shifts from dryland and uncultivated wasteland to paddy field have improved soil fertility and increased soil productivity, resulting in the present dominance of rice farming in the arable land of this region (Gong and Xu, 1990). Paddy soils in the region have an increasing organic C content, which may be in part related to the fact that high clay content and strong acidity in the soils slow the decomposition of SOM (Lin et al., 1995; Li et al., 2000) and that there is a high amount of crop residue returned to soil under double crop rice. On the other hand, dynamics of C and N cycling in paddy soils of this region could be responsible for those high SOC contents, and clarification of these dynamics will help understand regional roles of soil as a sink or source of atmospheric CO2. While close relationships between DOM and SOM mineralization have been most recently well documented, few studies have quantified the contribution of DOM to soil C and N mineralization, especially in agricultural soils such as paddy fields. The objective of this study was to determine the correlation between DOM and soil fertility and investigate effects of DOM removal on organic C and N mineralization in paddy soils in order to assess the contribution of DOM to C and N cycling in paddy soils of subtropical China.
2. Materials and methods
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physical and chemical analyses. The basic physical and chemical properties of the soils tested are presented in Table 1. 2.2. Experimental methods The differences in C, N mineralization between the original soils and those after DOM removal under anaerobic incubation were considered to be attributed to the removal of DOM. 2.2.1. Removal of soil DOM Fresh moist soils passing through a 2-mm sieve were weighed. The soil was shaken using reciprocating shaker for 30 min with double distilled water (2:1 of water–soil ratio) and centrifuged for 20 min (4000 rpm). The extracts were removed and passed through a 0.45 µm cellulose acetate membrane filter for determination of DOC and DON concentration (Liang et al., 1996, 1998). The DOM-removed soils were then pre-incubated for 1 week under indoor temperature. 2.2.2. C mineralization Carbon mineralization was estimated by measuring CO2 evolution from soils over a 20-day period (Goyal et al., 1999). Three portions of fresh moist original soil or DOM-removed soil (50 g, dry soil basis) were placed into 500-ml capacity jars and mixed with double distilled water using 2:1 of water–soil ratio. A test tube (6 cm length, 2.5 cm diameter) containing 10 ml of 0.1 mol/L NaOH was placed in each jar. The jars were sealed with rubber corks and incubated at 28 ± 1 °C. The CO2 trapped in NaOH solution was measured once 2 or 3 days during incubation by back titration with 0.1 mol/L HCl after addition of 2 ml 1 mol/L BaCl2 solution.
2.1. Study site and soil collection Soil samples were collected from paddy fields located at the Ecological Experimental Station of Red Soil, Chinese Academy of Sciences (116°5′30″E, 28°5′30″N). The station has a mean annual temperature of 17 °C, annual precipitation of 1785 mm (mainly during March and June), annual evaporation of 1318 mm, and a frost-free period of 261 days. The fields have various levels of soil fertility and productivity. Soils were derived from Tertiary sandstone, Quaternary red clay and river alluvium, classified respectively as Fe-leach-Stagnic Anthrosols, Fe-accumul-Stagnic Anthrosols and Hapli-Stagnic Anthrosols in Chinese Soil Taxonomy (Gong et al., 2007). The major cropping system is double rice cropping, managed under flood irrigation and NPK fertilizer application. Early rice straw was incorporated into the soil after harvesting. An adjacent upland soil derived from the same parent material was selected as a control. Composite soil samples (0–20 cm) in selected fields were taken in Nov 2005 and Jan, Apr, July and Nov 2006, representing different seasons and rice growing periods. The bulk samples were thoroughly mixed and passed through a 2-mm sieve after plant roots were removed and stored at field moisture content at 4 °C for DOM extraction and incubation within 1 week. Subsamples were air-dried and ground for
2.2.3. N mineralization Three portions of fresh moist original soil or DOM-removed soil (20 g, dry soil basis) were placed into 150-ml capacity jars and mixed with double distilled water (2:1 of water–soil ratio). The jars were sealed with rubber corks and incubated at 28 ± 1 °C. After 30 days, the water–soil ratio was adjusted to 5:1 and crystalline KCl was added to concentration of 2 mol/L, shaking for 1 h and filtering. Inorganic − nitrogen (NH+ 4 plus NO3 ) in the extracts was measured by flow injection analyser ( Astoria-Pacific Asta Co. LTD, USA). A separated set of soils was also extracted at the beginning of incubation to determine initial levels of inorganic N. Nitrogen mineralization was determined by subtracting initial inorganic N values from those after 30 days of incubation (Keeney and Nelson, 1982). 2.2.4. Analytic methods Soil organic C was determined by wet digestion of the Tyurin method (Lu, 1999), and total soil N was measured by the Kjeldahl method (Bremner and Mulvancy, 1982), and pH by the pH potentiometric method using distilled water (2.5:1 of water–soil ratio). The composition of soil particle sizes was analyzed with the pipette method (Liu, 1996). DOC in the extract was determined by total organic carbon (TOC)
Table 1 Some physical and chemical properties of soils tested. Symbol
Soil type
Parent material
Fertility level
pH (H2O)
Org C (g/kg)
Total N (g/kg)
Particle sizes (g/kg) 2–0.05 mm
0.05–0.002 mm
b 0.002 mm
HT LT UT HQ MQ LQ UQ RA
Paddy soil Paddy soil Upland soil Paddy soil Paddy soil Paddy soil Upland soil Paddy soil
Tertiary sandstone Tertiary sandstone Tertiary sandstone Quaternary red clay Quaternary red clay Quaternary red clay Quaternary red clay River alluvium
High Low Middle High Middle Low Middle High
5.19 5.26 5.91 5.22 5.33 5.19 5.26 5.22
17.1 6.93 5.77 23.0 11.4 8.48 6.60 16.4
1.76 0.99 0.67 2.48 1.40 0.94 0.91 2.13
529.0 536.3 600.5 310.8 217.1 265.2 297.5 374.7
333.3 245.7 256.6 515.6 455.4 371.7 297.0 523.2
137.7 218.0 142.9 173.6 327.5 363.1 405.5 102.1
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autoanalyser (TOC-5000A). DON in extracts was determined by − subtracting inorganic N (NH+ 4 plus NO3 ) from total dissolved N + − (TDN). NH4 , NO3 and TDN were measured by flow injection analyzer. Adsorption values of DOM were determined using ultraviolet–visible spectrophotometer in 280 nm after DOC concentration in extracts was diluted to 1 mg/L. 2.3. Statistic analysis The measurements of physical and chemical items were duplicated. Standard errors of the means of the three replicates from each soil were calculated. Variation among the soils was analyzed with SPSS 12.0 (SPSS Inc., 2003) and tested by Duncan's method and a significant level of 0.05, a priori, was chosen in all analyses. A logarithmic function was used to describe the changes in SOC mineralization rates with incubation time. 3. Results and analysis 3.1. DOM contents among soil types, fertility levels and seasons Soil types significantly controlled DOC contents in Nov 2005 (Fig. 1A). DOC contents in paddy soils were obviously higher than those in upland soils. In paddy soils, DOC contents ranged from 15.5 mg/ kg to 68.7 mg/kg with an average of 41.7 mg/kg. The corresponding values in upland soils were 10.3–12.0 mg/kg with an average of 11.2 mg/kg. DOC contents in paddy soils were also affected by the fertility. Soils with high fertility had 2.2 and 3.7 times higher values than
those with middle fertility and low fertility (significant difference at p b 0.01). It was noted that paddy fields with high fertility derived from both Tertiary sandstone and Quaternary red clay had 2.5 and 2.7 times higher SOC contents than those with low fertility, while 4.4 and 3.2 times higher DOC contents, showing DOC as a sensitive indicator of soil fertility change. Among parent materials, paddy soil with high fertility from Tertiary sandstone had 12.2% higher DOC content than that from Quaternary red clay. DON contents in soils were in the order: paddy soils with high fertility N middle fertilityN low fertilityN upland red soil. DON contents in paddy soils with high fertility derived from three parent materials were on average 17.4 mg/kg, which was 1.2, 1.5 and 2.0 times higher than that in paddy soils with middle and low fertility and upland red soil, respectively (Fig. 1B). Statistical analyses showed that a significant difference existed in DON contents between paddy soils with high fertility and low fertility and upland red soils (p b 0.01). Fig. 1B shows that at the same fertility level, DON contents were similar in paddy soils from both Tertiary sandstone and Quaternary red clay, indicating that parent materials had no significant effect on soil DON content. During experimental periods, DOC contents in tested soils varied from 3.8 to 68.7 mg/kg, which had the highest value in Nov and the lowest in July (Fig. 1A). The soil DOC contents in Nov 2005 and July 2006 were 10.3–68.7 mg/kg (average, 34.1 mg/kg) and 3.8–32.0 mg/kg (average, 14.8 mg/kg), respectively. The average DOC in the summer of 2006 was only 43% of that in the winter. Results also showed obvious differences in seasonal variation of DOC among soils. In soils derived from Tertiary sandstone, variation coefficients of DOC contents in paddy soils with high and low fertility and upland soil were 34%, 33% and 43%,
Fig. 1. Seasonal dynamics of DOC (A, B) and DON (C, D) in different soils of subtropical China. Vertical bars indicate the standard error of the averages. HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
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respectively. In soils derived from Quaternary red clay, the variation coefficient of DOC contents in paddy soil with high fertility was 24%, while those in the paddy soils with middle and low fertility and the upland soil were respectively 39%, 45% and 36%. This indicates that seasonal variation was more significant in the middle and low fertility paddy soils and the upland red soils. DON in soils has similar seasonal variation to DOC (Fig. 1B). DON contents in tested soils varied from 2.9 to 18.3 mg/kg (average, 6.8 mg/ kg), with the highest value of 18.3 mg/kg in Nov and the lowest 2.9 mg/ kg in July. The average content of DON in soils in July 2006 was only 50% of that in Nov 2005. Variation coefficients of DON contents in paddy soils with high fertility were lower than those in paddy soils with low fertility and upland red soils. In soils derived from Tertiary sandstone, high fertility paddy soil had 22% of the variation coefficient of DON contents, and the corresponding values for low fertility paddy soil and upland soil were 27% and 37%, respectively. In soils derived from Quaternary red clay, the variation coefficient of DON content in high fertility paddy soil was 18%, which was only 70% and 66% of those in low fertility paddy soil and upland soil, respectively. This suggests that seasonal variation of DON contents decreased with soil fertility improvement. 3.2. Effect of DOM removal on SOC mineralization Fig. 2 shows changes in SOC mineralization rates of different soil types during incubation periods. A similar tendency in changes in organic C mineralization with incubation days was observed between the original soils and the DOM-removed soils. Mineralization rates of soil organic C were higher in the early period of incubation and then decreased rapidly. After a period of 8 days, organic C mineralization rates in the original soils and the DOM-removed soils were 48.7%– 66.4% (average, 56%) and 49%–73% (average, 59%) of those at the beginning of incubation, respectively. 12 days after the trail start date SOC mineralization rates decreased slightly further and were maintained at about 50% of those in the first day of incubation. Generally speaking, changes in soil organic C mineralization rates with incubation time followed a logarithmic function (Table 2), where Y was organic C mineralization rates (CO2, ml/kg d) and X was incubation days (d). During 20 days' incubation, DOM removal decreased both rates and cumulative amounts of SOC mineralization. Results in Fig. 2 show that in the early stage of incubation, mineralization rates of organic C in DOM-removed soils were significantly lower than those in original soils. However, the difference in organic C mineralization rates between original soils and DOM-removed soils reduced with the
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Table 2 Simulation equations for mineralization of SOC during incubation period as affected by the removal of DOM. Soils Equations coefficients of determination
HT LT UT HQ MQ LQ UQ RA
Original soils
DOM-removed soils
Y = −7.037Ln(X) + 32.33 (R2 = 0.907**) Y = −2.725Ln(X)+16.26 (R2 = 0.911**) Y = −2.053Ln(X)+12.48 (R2 = 0.924**) Y = −9.292Ln(X) + 38.66 (R2 = 0.974**) Y = −10.23Ln(X) + 42.73 (R2 = 0.989**) Y = −3.419Ln(X)+17.59 (R2 = 0.931**) Y = −1.959Ln(X)+10.41 (R2 = 0.835*) Y = −11.52Ln(X) + 45.01 (R2 = 0.968**)
Y = −5.502Ln(X)+28.15 (R2 = 0.856*) Y = −1.994Ln(X)+13.92 (R2 = 0.776*) Y = −1.272Ln(X)+10.02 (R2 = 0.889**) Y = −7.568Ln(X) + 34.01 (R2 = 0.972**) Y = −8.169Ln(X) + 36.98 (R2 = 0.976**) Y = −2.484Ln(X)+15.10 (R2 = 0.894**) Y = −1.4791Ln(X) + 8.93 (R2 = 0.768) Y = −9.236Ln(X) + 38.86 (R2 = 0.939**)
Notes: In simulation equations, Y was organic C mineralization rates (CO2, ml/kg d) and X was incubation days (d). HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
period of incubation and was insignificant at the end of incubation. At the beginning of incubation, organic C mineralization rates in original soils and DOM-removed soils were 10.0–38.1 ml/kg d and 8.8– 33.8 ml/kg d, respectively, the difference was in the range of 1.2– 4.7 ml/kg d with an average of 2.7 ml/kg d. At the 8th day, the difference reduced to 0.2–2.0 ml/kg d with an average of 0.9 ml/kg d, which was 25–65% (average, 33%) of that in the beginning. After 12 days' incubation, no significant difference in organic C mineralization rates was found between original soils and DOM-removed soils. After 12 days' incubation, cumulative mineralization of organic C in original soils was 78.8–285.6 ml/kg, and in DOM-removed soils was 71.8–260.6 ml/kg. This represents a decrase of 7.0%–10.7% (average, 8.1%) compared with those in original soils. At the end of incubation (Fig. 3), cumulative mineralization of organic C in DOM-removed soils declined by 3.5%–9.0% (average, 5.4%). T-tests showed that the difference in cumulative amounts of organic C mineralization between the original soils and the DOM-removed soils was significant (p b 0.05) in the first 12 days of incubation, but that there was no
Fig. 2. Mineralization rates of SOC in subtropical China during incubation period (sampled in Nov 2005). HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
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Fig. 3. Cumulative amounts of C mineralized from soils in subtropical China during 20 days' incubation (sampled in Nov 2005). Vertical bars indicate the standard error of the averages. HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
significance during the whole period. The reduction in cumulative amounts of organic C mineralization caused by DOM removal was similar between paddy soils with high fertility from two kinds of parent materials (4.1% from Tertiary sandstone and 4.5% from Quaternary red clay). However, in paddy soils with low fertility, a greater decrease (6.7%) in cumulative amounts of organic C mineralization by DOM removal was observed in Tertiary sanstone than in Quaternary red clay (3.5%). 3.3. Effect of DOM removal on SON mineralization Soil N mineralization is closely related to N supply. In terrestrial ecosystems, mineralization is an important process to provide N and other nutrients for crop growth. Changes in soil N mineralization were expressed by the function of SON content, biological availability, hydro-thermal condition and mineralization duration etc. (Zhu and Wen, 1992). During 30 days' incubation, cumulative mineralization of organic N in tested soils was between 12.2 and 50.3 mg/kg and mineralization rates shifted from 1.6% to 2.2% over the 30 days. The cumulative mineralization of organic N increased with paddy soil fertility level ( high N middle N low ), and the values were higher in paddy soils than in upland soils. The reason is that soil fertility determined microbial community and microorganism populations, which drived SON mineralization (Haynes, 2000). DOM significantly influenced mineralization of SON. DOM removal decreased significantly the cumulative mineralization of organic N in soils (Fig. 4). During the incubation period the cumulative mineralization of organic N in DOM-removed soils was 2.2–8.7 mg/kg lower than those in original soils (decreased by 13.6%–27.4% with an average of 18.8%). T-tests showed that, except for Quaternary paddy soil with low fertility, the difference in cumulative N mineralization was significant (p b 0.05) between original soils and DOM-removed soils. This indicates that DOM plays an important role in organic N mineralization of paddy soils in subtropical China. In high fertility level soils, effects of DOM removal were not significant between soils with various parent materials. However, a significant difference was observed between soils with various parent materials when soils had low fertility. In low fertility level soils, DOM removal decreased the cumulative mineralization of organic N by 27.4% in paddy soil from Tertiary sandstone and only 13.6% from Quaternary red clay (p b 0.05). Results also showed that the decline in cumulative mineralization of
Fig. 4. Cumulative amounts of N mineralized from soils in subtropical China during 30 days' incubation (sampled in Nov 2005). Vertical bars indicate the standard error of the averages. HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
SON by DOM removal accounted for 21%–52% of DON contents, which indicates that this part of DON had higher bioavailability and played a key role in mineralization of SON. 3.4. Seasonal variation versus DOM removal effects on soil C and N mineralization Nature and composition of DOM were the important factors determining the function and behavior of DOM in soil ecosystem. Over seasons, soil moisture, temperature and crop growth status resulted in a large variation in content and chemical composition of DOM, which influenced its roles in soil C and N mineralization. Decline in cumulative mineralization of SOC by DOM removal was affected by soil types and seasons (Fig. 5). In soils derived from Tertiary sandstone and river alluvium, DOM removal decreased cumulative mineralization of organic C by 1.6%–20.3% with an average of 7.1% and caused a seasonal variation. The decline in cumulative mineralization of organic C by DOM removal was low in Nov 2005, then rapidly increased and reached the maximum (13.4%) in Jan 2006. Afterwards, the decline decreased quickly and reached the minimum (2.9%) in July, and then changed slightly. Comparatively, changes in organic C mineralization of soils from Quaternary red clay by DOM removal had less seasonal variation (4.5%). DOM removal significantly decreased cumulative mineralization of SON and the reduction was strongly affected by seasons (Fig. 6). After DOM removal, declines in the cumulative N mineralization in soils were the highest in Nov 2005, low in Jan and April 2006, and the lowest in July 2006. But, it increased to a larger value in Nov 2006. For example, in paddy soils with low fertility from Tertiary sandstone, DOM removal decreased cumulative mineralization of organic N in soils by 27.3% in Nov 2005, 18.2% in Jan 2006, 18.4% in April 2006, and 14.6% in July 2006. 4. Discussions DOM content was not only significantly affected by land use patterns, but also by seasonal variation (Mcdowell and Wood, 1984; Embacher et al., 2007; Fujii et al., 2009). In this study, DOM content varied with soil types, being higher in paddy soils than those in upland soils. Paddy soils with high fertility had significantly higher DOM content than those with low fertility. In rice cultivation of subtropical
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Fig. 5. Seasonal variations of the decreases in cumulative mineralization of SOC after DOM removal. HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
China, large amounts of crop residues and organic manures returned to the field caused high SOC and DOC contents in paddy soils (Li, 1999; Li et al., 2005). Comparatively, poor crop growth and management level and small organic material input in the uplands resulted in low SOC and DOC contents in red soils. In addition, long-term submergence periods in rice cultivation enhanced solubilization of organic matter and depolymerization of aggregates, which also increased DOM contents in paddy soils (Li et al., 2004). DOM contents were the highest in Nov, and the lowest in July. This may be in part explained by the fact that after rice harvest in Oct, temperature and moisture conditions were suitable for microorganism activity and the dead roots of rice were decomposed to form low molecular organic compounds, which increased soil DOM contents in Nov. In July, the highest temperature and strong microorganism activity increased significantly decomposition and loss of DOM (Scott et al., 1998). Due to its higher bioavailability, soil DOM was the important energy source for microbial growth and decomposition. However, relationships between DOM and SOC mineralization were discrepantly reported in literature. Some studies had indicated that the content and turnover rate of soil DOM directly affected microbial composition and activity, which drove mineralization of SOC (Haynes, 2000). In many soils, DOM content related significantly to CO2–C release from mineralization (Marschner and Bredow, 2002; Lou et al., 2004; Rees and Arker, 2005; Van Hees et al., 2005). However, in an
incubation experiment, Cook and Allan (1992) found that mineralization rate of SOC gradually reduced with incubation time, but the DOM content in soil remained stable or even increased. Studies on agricultural soils in California by Lundquist et al. (1999) pointed out that there was no direct relationship between DOM content and SOC mineralization and DOM might not be the available component for microbial growth. In the present study, results showed that DOM removal significantly decreased cumulative mineralization of organic C in soils derived from Tertiary sandstone and river alluvium. But such a relationship was mainly observed in Jan and April. In soils derived from Quaternary red clay, although mineralization rates and cumulative mineralization of organic C were decreased by DOM removal in all seasons, the overall relationship was not significant. Effects of DOM removal on C mineralization might be due to DOM composition in soils. It has been reported that E280 value of DOM was related to its aromatic compounds (Kalbitz et al., 2003). Higher E280 value meant that DOM contained more aromatic compounds with complicated structure and low availability. Our analytical results showed that E280 values of DOM in soils from Quaternary red clay were 0.030–0.056 with an average of 0.037 in samples taken in different seasons (Fig. 7). In soils from Tertiary sandstone and river alluvium, E280 values of DOM in Nov were higher than 0.033, which were 2.1 and 1.5 times of those in Jan and April, respectively. These results indicated that DOM in the soils contained more aromatic substances and had lower
Fig. 6. Seasonal variations of the decreases in cumulative mineralization of SON after DOM removal. HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
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Fig. 7. Seasonal changes in absorption values of DOM at 280 nm. HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
bioavailability. Therefore, it may play a minor role in soil C mineralization. In addition, microbial communities and metabolism were shifted within seasons and soil types. In a specific environmental condition, microbes might degrade more solid-phase organic matter into DOM with small molecular weight via extracellular enzyme, resulting in an uncertain relationship between carbon source for microbial growth and DOM availability. It should be noted that in July, E280 values of DOM in soils from Tertiary sandstone and river alluvium were only 0.018–0.033 (average, 0.024), while the effect of DOM on soil C mineralization was very small. This might be due to the lowest DOC content in the season. At present, there have been numerous reports about SON nitrification mechanism, but less attention is paid on transformation from DON to NH+ 4 –N in agricultural soils (Murphy et al., 2000). In this study, DOM removal significantly decreased cumulative mineralization of SON in all seasons, which was different from that on mineralization of SOC. This was probably because of more significance of DON in transformation of SON. Although soil microorganism can directly transform organic N into NH+ 4 –N by transaminase, the main approach of SON mineralization is through the path where non-DON is decomposed or depolymerized into DON by microbial extracellular enzyme and then DON with small molecular weight entered microbial cell for next step catabolism (Zaman et al., 1999). Nitrogen status in microbial biomass decided mineralization and fixation of soil N. Therefore, DON plays a role of intermediate N pool during mineralization of SOM and is regarded as initial product of SON mineralization to modulate NH+ 4 –N supply and bio-transformation of organic N. The effect of DOM removal on mineralization of SON had obvious seasonal variation. After DOM removal, declines in organic N cumulative mineralization in soils were the highest in Nov, followed by those in Jan and April, and the lowest in July, which was in agreement with dynamics of soil DON contents. This indicates that DON content was an important factor determining its role in SON mineralization. Organic matter decomposition produced soluble organic compounds, however, not all of these organic compounds were further degraded by microorganisms and maybe DON was partly mineralized. Some previous research has showen that DON is composed of easily- and difficultly-degraded components (Zaman et al., 1999; Yu et al., 2002). Turnover time of the former was 1 to 5 days, while the latter was 80 days to 9 years. Results in this study also confirm that SON mineralization has a close relationship with an active part of DON. 5. Conclusions DOM contents in soils were significantly affected by fertility levels and land use patterns. DOC and DON contents in soils changed in
order of the paddy soil with high fertility N middle fertility N low fertility N the upland red soil. Both increased generally with improvement of soil fertility. Effects of DOM on C and N mineralization were dependant upon soil types. DOM removal significantly reduced cumulative mineralization of organic C in soils from Tertiary sandstone and river alluvium in Jan and April, but had no significant effects in soils derived from Quaternary red clay. DOM had a significant impact on SON mineralization. DOM removal significantly decreased cumulative mineralization of organic N in tested soils and the declines were characterized by seasonal variations. This indicates that, although DOM only accounts for a small portion of SOM, it was an important N source for microbial activities and played an important role in N transformation in paddy soils of subtropical China. Acknowledgements This study is jointly supported by funding from the National Basic Research Program of China (no. 2007CB109301) and the National Natural Science Foundation of China (no. 40871122). We thank Mrs. Y.P. Che for her technical help. References Bremner, J.M., Mulvancy, C.S., 1982. Nitrogen-total. In: Page, A.L., Miller, R.H., Keeney, D. R. (Eds.), Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties. American Society of Agronomy, Madison, USA, pp. 595–624. Burford, J., Bremner, J.M., 1975. Relationships between the denitrification capacities of soils and total, water-soluble and readily decomposable soil organic matter. Soil Biol. Biochem. 7, 389–394. Cook, B.D., Allan, D.L., 1992. Dissolved organic carbon in old field soils: compositional changes during the biodegradation of soil organic matter. Soil Biol. Biochem. 24, 595–600. Cookson, W.R., Murphy, D.V., 2004. Quantifying the contribution of dissolved organic matter to soil nitrogen cycling using 15N isotopic pool dilution. Soil Biol. Biochem. 36, 2097–2100. Christ, M.J., David, M.B., 1996. Dynamics of extractable organic carbon in spodosol forest floors. Soil Biol. Biochem. 28, 1171–1179. Davidson, E.A., Galloway, L.F., Strand, M.K., 1987. Assessing available carbon: comparison of techniques across selected forest soils. Commun. Soil Sci. Plant Anal. 18, 45–64. Du, Q.L., He, K. (Eds.), 2001. Almanac of China's Agriculture 2000. China Agriculture Press, Beijing. Ellert, B.H., Gregorich, E.G., 1995. Management-induced changes in the actively cycling fractions of soil organic matter. In: Mcfee, W.W., Kelly, J.M. (Eds.), Carbon Forms and Functions in Forest Soils. Soil Science Society of America, Inc., Wisconsin, Madison, USA, pp. 119–138. Embacher, A., Zsolny, A., Gattinger, A., Munch, J.C., 2007. The dynamics of waterextractable organic matter (WEOM) in common arable topsoils: I. Quantity, quality and function over a three year period. Geoderma 139, 11–22. Fujii, K., Uemura, M., Hayakawa, C., Funakawa, S., Kosaki, T., Ohta, S., 2009. Fluxes of dissolved organic carbon in two tropical forest ecosystems of East Kalimantan, Indonesia. Geoderma 152, 127–136.
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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a
Organic C and N mineralization as affected by dissolved organic matter in paddy soils of subtropical China Z.P. Li a,b,⁎, C.W. Han a,b, F.X. Han c a b c
State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China Graduate University of the Chinese Academy of Sciences, Beijing 100049, China Institute for Clean Energy Technology, Mississippi State University, 205 Research Blvd, Starkville, MS 39759, USA
a r t i c l e
i n f o
Article history: Received 6 August 2009 Received in revised form 10 April 2010 Accepted 20 April 2010 Available online 20 May 2010 Keywords: DOM dynamics DOM removal Mineralization of C N Incubation Paddy soils Subtropical China
a b s t r a c t In this study, dynamics of dissolved organic matter (DOM) and mineralization of soil organic C and N in subtropical China were investigated. Paddy and upland soils were derived from Tertiary sandstone, Quaternary red clay and river alluvium with low, middle, and high levels of soil fertility. Dissolved organic carbon contents (DOC) varied from 3.8 to 68.7 mg/kg and dissolved organic nitrogen contents (DON) ranged from 2.9 to 18.3 mg/kg, respectively, during the experimental period. Both DOC and DON increased with increasing soil fertility. The content of DOM was characterized by strong seasonal fluctuation in soils, and seasonal patterns showed the maximum value in Nov and the minimum in July. The contribution of DOM to mineralization of C and N was significantly affected by soil types and seasons. DOM removal significantly decreased the cumulative organic C mineralization in soils derived from Tertiary sandstone and River alluvium (by 1.6%–20.3% with an average of 7.1%), while it did not significantly change those in soils derived from Quaternary red clay. Similarly, the cumulative mineralization of N in paddy soils after DOM removal decreased by 6.7%–27.3% over seasons. In this study, DOM played an important role in soil C and N mineralization in subtropical China. It appears that contribution of DOM to minerilization of C and N in paddy soils was possibly related to DOM content and composition. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Mineralization of soil organic matter (SOM) is an important biochemical process related closely to release and supply of plant nutrients, formation and emission of greenhouse gases, and maintenance of soil quality. A better understanding of the dynamics and mechanisms of SOM mineralization is essential so that best management practices are adopted to scientifically manage soil nutrients and mitigate global warming. SOM mineralization is a microbial process and affected by temperature, moisture, soil physical and chemical properties, and agricultural practices (Parton et al., 1987; Wadman and de Haan, 1997; Raiesi, 2006). In addition, chemical composition and nature of SOM is a critical factor for decomposition (Levi-Minzi et al., 1990; Nelson et al., 1994). SOM consists of different biologically-available compounds or fractions. Solid phase is expected to be less susceptible to decomposition while DOM is easily decomposable (Ellert and Gregorich, 1995). Generally, depolymerization and solubilization of SOM were considered to be a prerequisite to mineralization ⁎ Corresponding author. State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China. Tel.: + 86 25 86881505; fax: + 86 25 86881000. E-mail address: [email protected] (Z.P. Li). 0016-7061/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2010.04.015
(Marschner and Kalbitz, 2003). These processes release nutrients to soil solution, which are available to the soil microbial biomass and plants. The soil solution is a bottleneck in the mineralization convertion of solid-phase organic matter to CO2 and CH4 (Marschner and Kalbitz, 2003; Qiu et al., 2008). Therefore, dynamics of DOM play an important role in mineralization of SOM (Gregorich et al., 1998; Motavalli et al., 2000). Numerous studies have examined the pool size and chemical composition of DOM (Christ and David, 1996; Michalzik and Matzner, 1999; Kalbitz et al., 2000; Embacher et al., 2007), while the role of DOM in the microbial mediated mineralization of SOM remains unclear. Burford and Bremner (1975) and Davidson et al. (1987) found a strong correlation between DOC concentration and soil organic carbon (SOC) mineralization in a variety of soil types. However, Cook and Allan (1992) reported that there was no obvious relationship between DOC and instantaneous rates of SOC mineralization over a long incubation period. Liang et al. (1996) suggested that application of high amounts of soluble C (e.g., extracts of composted manure) could accelerate indigenous soil C decomposition. Cookson and Murphy (2004) quantified the importance of DOM in soil C and N cycling. They showed that 2 weeks after DOM removal microbial respiration from soils was not altered, but significant declines in microbial biomass N, potentially mineralizable N, gross N mineralization and gross nitrification occurred. They concluded up to 25% of microbial N supply from DON. Jones and Kielland (2002)
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and Jones et al. (2004) proposed that the conversion of insoluble organic + − N to DON, rather than DON to NH+ 4 or NH4 to NO3 actually was a major constraint to N supply. About 93% of paddy field in China is distributed in the tropical and subtropical regions (Li, 1992). Rice cultivation areas have steadily increased in subtropical China in the past decades (Du and He, 2001). The shifts from dryland and uncultivated wasteland to paddy field have improved soil fertility and increased soil productivity, resulting in the present dominance of rice farming in the arable land of this region (Gong and Xu, 1990). Paddy soils in the region have an increasing organic C content, which may be in part related to the fact that high clay content and strong acidity in the soils slow the decomposition of SOM (Lin et al., 1995; Li et al., 2000) and that there is a high amount of crop residue returned to soil under double crop rice. On the other hand, dynamics of C and N cycling in paddy soils of this region could be responsible for those high SOC contents, and clarification of these dynamics will help understand regional roles of soil as a sink or source of atmospheric CO2. While close relationships between DOM and SOM mineralization have been most recently well documented, few studies have quantified the contribution of DOM to soil C and N mineralization, especially in agricultural soils such as paddy fields. The objective of this study was to determine the correlation between DOM and soil fertility and investigate effects of DOM removal on organic C and N mineralization in paddy soils in order to assess the contribution of DOM to C and N cycling in paddy soils of subtropical China.
2. Materials and methods
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physical and chemical analyses. The basic physical and chemical properties of the soils tested are presented in Table 1. 2.2. Experimental methods The differences in C, N mineralization between the original soils and those after DOM removal under anaerobic incubation were considered to be attributed to the removal of DOM. 2.2.1. Removal of soil DOM Fresh moist soils passing through a 2-mm sieve were weighed. The soil was shaken using reciprocating shaker for 30 min with double distilled water (2:1 of water–soil ratio) and centrifuged for 20 min (4000 rpm). The extracts were removed and passed through a 0.45 µm cellulose acetate membrane filter for determination of DOC and DON concentration (Liang et al., 1996, 1998). The DOM-removed soils were then pre-incubated for 1 week under indoor temperature. 2.2.2. C mineralization Carbon mineralization was estimated by measuring CO2 evolution from soils over a 20-day period (Goyal et al., 1999). Three portions of fresh moist original soil or DOM-removed soil (50 g, dry soil basis) were placed into 500-ml capacity jars and mixed with double distilled water using 2:1 of water–soil ratio. A test tube (6 cm length, 2.5 cm diameter) containing 10 ml of 0.1 mol/L NaOH was placed in each jar. The jars were sealed with rubber corks and incubated at 28 ± 1 °C. The CO2 trapped in NaOH solution was measured once 2 or 3 days during incubation by back titration with 0.1 mol/L HCl after addition of 2 ml 1 mol/L BaCl2 solution.
2.1. Study site and soil collection Soil samples were collected from paddy fields located at the Ecological Experimental Station of Red Soil, Chinese Academy of Sciences (116°5′30″E, 28°5′30″N). The station has a mean annual temperature of 17 °C, annual precipitation of 1785 mm (mainly during March and June), annual evaporation of 1318 mm, and a frost-free period of 261 days. The fields have various levels of soil fertility and productivity. Soils were derived from Tertiary sandstone, Quaternary red clay and river alluvium, classified respectively as Fe-leach-Stagnic Anthrosols, Fe-accumul-Stagnic Anthrosols and Hapli-Stagnic Anthrosols in Chinese Soil Taxonomy (Gong et al., 2007). The major cropping system is double rice cropping, managed under flood irrigation and NPK fertilizer application. Early rice straw was incorporated into the soil after harvesting. An adjacent upland soil derived from the same parent material was selected as a control. Composite soil samples (0–20 cm) in selected fields were taken in Nov 2005 and Jan, Apr, July and Nov 2006, representing different seasons and rice growing periods. The bulk samples were thoroughly mixed and passed through a 2-mm sieve after plant roots were removed and stored at field moisture content at 4 °C for DOM extraction and incubation within 1 week. Subsamples were air-dried and ground for
2.2.3. N mineralization Three portions of fresh moist original soil or DOM-removed soil (20 g, dry soil basis) were placed into 150-ml capacity jars and mixed with double distilled water (2:1 of water–soil ratio). The jars were sealed with rubber corks and incubated at 28 ± 1 °C. After 30 days, the water–soil ratio was adjusted to 5:1 and crystalline KCl was added to concentration of 2 mol/L, shaking for 1 h and filtering. Inorganic − nitrogen (NH+ 4 plus NO3 ) in the extracts was measured by flow injection analyser ( Astoria-Pacific Asta Co. LTD, USA). A separated set of soils was also extracted at the beginning of incubation to determine initial levels of inorganic N. Nitrogen mineralization was determined by subtracting initial inorganic N values from those after 30 days of incubation (Keeney and Nelson, 1982). 2.2.4. Analytic methods Soil organic C was determined by wet digestion of the Tyurin method (Lu, 1999), and total soil N was measured by the Kjeldahl method (Bremner and Mulvancy, 1982), and pH by the pH potentiometric method using distilled water (2.5:1 of water–soil ratio). The composition of soil particle sizes was analyzed with the pipette method (Liu, 1996). DOC in the extract was determined by total organic carbon (TOC)
Table 1 Some physical and chemical properties of soils tested. Symbol
Soil type
Parent material
Fertility level
pH (H2O)
Org C (g/kg)
Total N (g/kg)
Particle sizes (g/kg) 2–0.05 mm
0.05–0.002 mm
b 0.002 mm
HT LT UT HQ MQ LQ UQ RA
Paddy soil Paddy soil Upland soil Paddy soil Paddy soil Paddy soil Upland soil Paddy soil
Tertiary sandstone Tertiary sandstone Tertiary sandstone Quaternary red clay Quaternary red clay Quaternary red clay Quaternary red clay River alluvium
High Low Middle High Middle Low Middle High
5.19 5.26 5.91 5.22 5.33 5.19 5.26 5.22
17.1 6.93 5.77 23.0 11.4 8.48 6.60 16.4
1.76 0.99 0.67 2.48 1.40 0.94 0.91 2.13
529.0 536.3 600.5 310.8 217.1 265.2 297.5 374.7
333.3 245.7 256.6 515.6 455.4 371.7 297.0 523.2
137.7 218.0 142.9 173.6 327.5 363.1 405.5 102.1
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autoanalyser (TOC-5000A). DON in extracts was determined by − subtracting inorganic N (NH+ 4 plus NO3 ) from total dissolved N + − (TDN). NH4 , NO3 and TDN were measured by flow injection analyzer. Adsorption values of DOM were determined using ultraviolet–visible spectrophotometer in 280 nm after DOC concentration in extracts was diluted to 1 mg/L. 2.3. Statistic analysis The measurements of physical and chemical items were duplicated. Standard errors of the means of the three replicates from each soil were calculated. Variation among the soils was analyzed with SPSS 12.0 (SPSS Inc., 2003) and tested by Duncan's method and a significant level of 0.05, a priori, was chosen in all analyses. A logarithmic function was used to describe the changes in SOC mineralization rates with incubation time. 3. Results and analysis 3.1. DOM contents among soil types, fertility levels and seasons Soil types significantly controlled DOC contents in Nov 2005 (Fig. 1A). DOC contents in paddy soils were obviously higher than those in upland soils. In paddy soils, DOC contents ranged from 15.5 mg/ kg to 68.7 mg/kg with an average of 41.7 mg/kg. The corresponding values in upland soils were 10.3–12.0 mg/kg with an average of 11.2 mg/kg. DOC contents in paddy soils were also affected by the fertility. Soils with high fertility had 2.2 and 3.7 times higher values than
those with middle fertility and low fertility (significant difference at p b 0.01). It was noted that paddy fields with high fertility derived from both Tertiary sandstone and Quaternary red clay had 2.5 and 2.7 times higher SOC contents than those with low fertility, while 4.4 and 3.2 times higher DOC contents, showing DOC as a sensitive indicator of soil fertility change. Among parent materials, paddy soil with high fertility from Tertiary sandstone had 12.2% higher DOC content than that from Quaternary red clay. DON contents in soils were in the order: paddy soils with high fertility N middle fertilityN low fertilityN upland red soil. DON contents in paddy soils with high fertility derived from three parent materials were on average 17.4 mg/kg, which was 1.2, 1.5 and 2.0 times higher than that in paddy soils with middle and low fertility and upland red soil, respectively (Fig. 1B). Statistical analyses showed that a significant difference existed in DON contents between paddy soils with high fertility and low fertility and upland red soils (p b 0.01). Fig. 1B shows that at the same fertility level, DON contents were similar in paddy soils from both Tertiary sandstone and Quaternary red clay, indicating that parent materials had no significant effect on soil DON content. During experimental periods, DOC contents in tested soils varied from 3.8 to 68.7 mg/kg, which had the highest value in Nov and the lowest in July (Fig. 1A). The soil DOC contents in Nov 2005 and July 2006 were 10.3–68.7 mg/kg (average, 34.1 mg/kg) and 3.8–32.0 mg/kg (average, 14.8 mg/kg), respectively. The average DOC in the summer of 2006 was only 43% of that in the winter. Results also showed obvious differences in seasonal variation of DOC among soils. In soils derived from Tertiary sandstone, variation coefficients of DOC contents in paddy soils with high and low fertility and upland soil were 34%, 33% and 43%,
Fig. 1. Seasonal dynamics of DOC (A, B) and DON (C, D) in different soils of subtropical China. Vertical bars indicate the standard error of the averages. HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
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respectively. In soils derived from Quaternary red clay, the variation coefficient of DOC contents in paddy soil with high fertility was 24%, while those in the paddy soils with middle and low fertility and the upland soil were respectively 39%, 45% and 36%. This indicates that seasonal variation was more significant in the middle and low fertility paddy soils and the upland red soils. DON in soils has similar seasonal variation to DOC (Fig. 1B). DON contents in tested soils varied from 2.9 to 18.3 mg/kg (average, 6.8 mg/ kg), with the highest value of 18.3 mg/kg in Nov and the lowest 2.9 mg/ kg in July. The average content of DON in soils in July 2006 was only 50% of that in Nov 2005. Variation coefficients of DON contents in paddy soils with high fertility were lower than those in paddy soils with low fertility and upland red soils. In soils derived from Tertiary sandstone, high fertility paddy soil had 22% of the variation coefficient of DON contents, and the corresponding values for low fertility paddy soil and upland soil were 27% and 37%, respectively. In soils derived from Quaternary red clay, the variation coefficient of DON content in high fertility paddy soil was 18%, which was only 70% and 66% of those in low fertility paddy soil and upland soil, respectively. This suggests that seasonal variation of DON contents decreased with soil fertility improvement. 3.2. Effect of DOM removal on SOC mineralization Fig. 2 shows changes in SOC mineralization rates of different soil types during incubation periods. A similar tendency in changes in organic C mineralization with incubation days was observed between the original soils and the DOM-removed soils. Mineralization rates of soil organic C were higher in the early period of incubation and then decreased rapidly. After a period of 8 days, organic C mineralization rates in the original soils and the DOM-removed soils were 48.7%– 66.4% (average, 56%) and 49%–73% (average, 59%) of those at the beginning of incubation, respectively. 12 days after the trail start date SOC mineralization rates decreased slightly further and were maintained at about 50% of those in the first day of incubation. Generally speaking, changes in soil organic C mineralization rates with incubation time followed a logarithmic function (Table 2), where Y was organic C mineralization rates (CO2, ml/kg d) and X was incubation days (d). During 20 days' incubation, DOM removal decreased both rates and cumulative amounts of SOC mineralization. Results in Fig. 2 show that in the early stage of incubation, mineralization rates of organic C in DOM-removed soils were significantly lower than those in original soils. However, the difference in organic C mineralization rates between original soils and DOM-removed soils reduced with the
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Table 2 Simulation equations for mineralization of SOC during incubation period as affected by the removal of DOM. Soils Equations coefficients of determination
HT LT UT HQ MQ LQ UQ RA
Original soils
DOM-removed soils
Y = −7.037Ln(X) + 32.33 (R2 = 0.907**) Y = −2.725Ln(X)+16.26 (R2 = 0.911**) Y = −2.053Ln(X)+12.48 (R2 = 0.924**) Y = −9.292Ln(X) + 38.66 (R2 = 0.974**) Y = −10.23Ln(X) + 42.73 (R2 = 0.989**) Y = −3.419Ln(X)+17.59 (R2 = 0.931**) Y = −1.959Ln(X)+10.41 (R2 = 0.835*) Y = −11.52Ln(X) + 45.01 (R2 = 0.968**)
Y = −5.502Ln(X)+28.15 (R2 = 0.856*) Y = −1.994Ln(X)+13.92 (R2 = 0.776*) Y = −1.272Ln(X)+10.02 (R2 = 0.889**) Y = −7.568Ln(X) + 34.01 (R2 = 0.972**) Y = −8.169Ln(X) + 36.98 (R2 = 0.976**) Y = −2.484Ln(X)+15.10 (R2 = 0.894**) Y = −1.4791Ln(X) + 8.93 (R2 = 0.768) Y = −9.236Ln(X) + 38.86 (R2 = 0.939**)
Notes: In simulation equations, Y was organic C mineralization rates (CO2, ml/kg d) and X was incubation days (d). HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
period of incubation and was insignificant at the end of incubation. At the beginning of incubation, organic C mineralization rates in original soils and DOM-removed soils were 10.0–38.1 ml/kg d and 8.8– 33.8 ml/kg d, respectively, the difference was in the range of 1.2– 4.7 ml/kg d with an average of 2.7 ml/kg d. At the 8th day, the difference reduced to 0.2–2.0 ml/kg d with an average of 0.9 ml/kg d, which was 25–65% (average, 33%) of that in the beginning. After 12 days' incubation, no significant difference in organic C mineralization rates was found between original soils and DOM-removed soils. After 12 days' incubation, cumulative mineralization of organic C in original soils was 78.8–285.6 ml/kg, and in DOM-removed soils was 71.8–260.6 ml/kg. This represents a decrase of 7.0%–10.7% (average, 8.1%) compared with those in original soils. At the end of incubation (Fig. 3), cumulative mineralization of organic C in DOM-removed soils declined by 3.5%–9.0% (average, 5.4%). T-tests showed that the difference in cumulative amounts of organic C mineralization between the original soils and the DOM-removed soils was significant (p b 0.05) in the first 12 days of incubation, but that there was no
Fig. 2. Mineralization rates of SOC in subtropical China during incubation period (sampled in Nov 2005). HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
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Fig. 3. Cumulative amounts of C mineralized from soils in subtropical China during 20 days' incubation (sampled in Nov 2005). Vertical bars indicate the standard error of the averages. HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
significance during the whole period. The reduction in cumulative amounts of organic C mineralization caused by DOM removal was similar between paddy soils with high fertility from two kinds of parent materials (4.1% from Tertiary sandstone and 4.5% from Quaternary red clay). However, in paddy soils with low fertility, a greater decrease (6.7%) in cumulative amounts of organic C mineralization by DOM removal was observed in Tertiary sanstone than in Quaternary red clay (3.5%). 3.3. Effect of DOM removal on SON mineralization Soil N mineralization is closely related to N supply. In terrestrial ecosystems, mineralization is an important process to provide N and other nutrients for crop growth. Changes in soil N mineralization were expressed by the function of SON content, biological availability, hydro-thermal condition and mineralization duration etc. (Zhu and Wen, 1992). During 30 days' incubation, cumulative mineralization of organic N in tested soils was between 12.2 and 50.3 mg/kg and mineralization rates shifted from 1.6% to 2.2% over the 30 days. The cumulative mineralization of organic N increased with paddy soil fertility level ( high N middle N low ), and the values were higher in paddy soils than in upland soils. The reason is that soil fertility determined microbial community and microorganism populations, which drived SON mineralization (Haynes, 2000). DOM significantly influenced mineralization of SON. DOM removal decreased significantly the cumulative mineralization of organic N in soils (Fig. 4). During the incubation period the cumulative mineralization of organic N in DOM-removed soils was 2.2–8.7 mg/kg lower than those in original soils (decreased by 13.6%–27.4% with an average of 18.8%). T-tests showed that, except for Quaternary paddy soil with low fertility, the difference in cumulative N mineralization was significant (p b 0.05) between original soils and DOM-removed soils. This indicates that DOM plays an important role in organic N mineralization of paddy soils in subtropical China. In high fertility level soils, effects of DOM removal were not significant between soils with various parent materials. However, a significant difference was observed between soils with various parent materials when soils had low fertility. In low fertility level soils, DOM removal decreased the cumulative mineralization of organic N by 27.4% in paddy soil from Tertiary sandstone and only 13.6% from Quaternary red clay (p b 0.05). Results also showed that the decline in cumulative mineralization of
Fig. 4. Cumulative amounts of N mineralized from soils in subtropical China during 30 days' incubation (sampled in Nov 2005). Vertical bars indicate the standard error of the averages. HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
SON by DOM removal accounted for 21%–52% of DON contents, which indicates that this part of DON had higher bioavailability and played a key role in mineralization of SON. 3.4. Seasonal variation versus DOM removal effects on soil C and N mineralization Nature and composition of DOM were the important factors determining the function and behavior of DOM in soil ecosystem. Over seasons, soil moisture, temperature and crop growth status resulted in a large variation in content and chemical composition of DOM, which influenced its roles in soil C and N mineralization. Decline in cumulative mineralization of SOC by DOM removal was affected by soil types and seasons (Fig. 5). In soils derived from Tertiary sandstone and river alluvium, DOM removal decreased cumulative mineralization of organic C by 1.6%–20.3% with an average of 7.1% and caused a seasonal variation. The decline in cumulative mineralization of organic C by DOM removal was low in Nov 2005, then rapidly increased and reached the maximum (13.4%) in Jan 2006. Afterwards, the decline decreased quickly and reached the minimum (2.9%) in July, and then changed slightly. Comparatively, changes in organic C mineralization of soils from Quaternary red clay by DOM removal had less seasonal variation (4.5%). DOM removal significantly decreased cumulative mineralization of SON and the reduction was strongly affected by seasons (Fig. 6). After DOM removal, declines in the cumulative N mineralization in soils were the highest in Nov 2005, low in Jan and April 2006, and the lowest in July 2006. But, it increased to a larger value in Nov 2006. For example, in paddy soils with low fertility from Tertiary sandstone, DOM removal decreased cumulative mineralization of organic N in soils by 27.3% in Nov 2005, 18.2% in Jan 2006, 18.4% in April 2006, and 14.6% in July 2006. 4. Discussions DOM content was not only significantly affected by land use patterns, but also by seasonal variation (Mcdowell and Wood, 1984; Embacher et al., 2007; Fujii et al., 2009). In this study, DOM content varied with soil types, being higher in paddy soils than those in upland soils. Paddy soils with high fertility had significantly higher DOM content than those with low fertility. In rice cultivation of subtropical
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Fig. 5. Seasonal variations of the decreases in cumulative mineralization of SOC after DOM removal. HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
China, large amounts of crop residues and organic manures returned to the field caused high SOC and DOC contents in paddy soils (Li, 1999; Li et al., 2005). Comparatively, poor crop growth and management level and small organic material input in the uplands resulted in low SOC and DOC contents in red soils. In addition, long-term submergence periods in rice cultivation enhanced solubilization of organic matter and depolymerization of aggregates, which also increased DOM contents in paddy soils (Li et al., 2004). DOM contents were the highest in Nov, and the lowest in July. This may be in part explained by the fact that after rice harvest in Oct, temperature and moisture conditions were suitable for microorganism activity and the dead roots of rice were decomposed to form low molecular organic compounds, which increased soil DOM contents in Nov. In July, the highest temperature and strong microorganism activity increased significantly decomposition and loss of DOM (Scott et al., 1998). Due to its higher bioavailability, soil DOM was the important energy source for microbial growth and decomposition. However, relationships between DOM and SOC mineralization were discrepantly reported in literature. Some studies had indicated that the content and turnover rate of soil DOM directly affected microbial composition and activity, which drove mineralization of SOC (Haynes, 2000). In many soils, DOM content related significantly to CO2–C release from mineralization (Marschner and Bredow, 2002; Lou et al., 2004; Rees and Arker, 2005; Van Hees et al., 2005). However, in an
incubation experiment, Cook and Allan (1992) found that mineralization rate of SOC gradually reduced with incubation time, but the DOM content in soil remained stable or even increased. Studies on agricultural soils in California by Lundquist et al. (1999) pointed out that there was no direct relationship between DOM content and SOC mineralization and DOM might not be the available component for microbial growth. In the present study, results showed that DOM removal significantly decreased cumulative mineralization of organic C in soils derived from Tertiary sandstone and river alluvium. But such a relationship was mainly observed in Jan and April. In soils derived from Quaternary red clay, although mineralization rates and cumulative mineralization of organic C were decreased by DOM removal in all seasons, the overall relationship was not significant. Effects of DOM removal on C mineralization might be due to DOM composition in soils. It has been reported that E280 value of DOM was related to its aromatic compounds (Kalbitz et al., 2003). Higher E280 value meant that DOM contained more aromatic compounds with complicated structure and low availability. Our analytical results showed that E280 values of DOM in soils from Quaternary red clay were 0.030–0.056 with an average of 0.037 in samples taken in different seasons (Fig. 7). In soils from Tertiary sandstone and river alluvium, E280 values of DOM in Nov were higher than 0.033, which were 2.1 and 1.5 times of those in Jan and April, respectively. These results indicated that DOM in the soils contained more aromatic substances and had lower
Fig. 6. Seasonal variations of the decreases in cumulative mineralization of SON after DOM removal. HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
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Fig. 7. Seasonal changes in absorption values of DOM at 280 nm. HT, LT and UT were respectively high, low fertility of paddy soils and upland soil derived from Tertiary sandstone. HQ, MQ, LQ and UQ were respectively high, middle, low fertility of paddy soils and upland soil derived from Quaternary red clay. RA was paddy soil derived from river alluvium.
bioavailability. Therefore, it may play a minor role in soil C mineralization. In addition, microbial communities and metabolism were shifted within seasons and soil types. In a specific environmental condition, microbes might degrade more solid-phase organic matter into DOM with small molecular weight via extracellular enzyme, resulting in an uncertain relationship between carbon source for microbial growth and DOM availability. It should be noted that in July, E280 values of DOM in soils from Tertiary sandstone and river alluvium were only 0.018–0.033 (average, 0.024), while the effect of DOM on soil C mineralization was very small. This might be due to the lowest DOC content in the season. At present, there have been numerous reports about SON nitrification mechanism, but less attention is paid on transformation from DON to NH+ 4 –N in agricultural soils (Murphy et al., 2000). In this study, DOM removal significantly decreased cumulative mineralization of SON in all seasons, which was different from that on mineralization of SOC. This was probably because of more significance of DON in transformation of SON. Although soil microorganism can directly transform organic N into NH+ 4 –N by transaminase, the main approach of SON mineralization is through the path where non-DON is decomposed or depolymerized into DON by microbial extracellular enzyme and then DON with small molecular weight entered microbial cell for next step catabolism (Zaman et al., 1999). Nitrogen status in microbial biomass decided mineralization and fixation of soil N. Therefore, DON plays a role of intermediate N pool during mineralization of SOM and is regarded as initial product of SON mineralization to modulate NH+ 4 –N supply and bio-transformation of organic N. The effect of DOM removal on mineralization of SON had obvious seasonal variation. After DOM removal, declines in organic N cumulative mineralization in soils were the highest in Nov, followed by those in Jan and April, and the lowest in July, which was in agreement with dynamics of soil DON contents. This indicates that DON content was an important factor determining its role in SON mineralization. Organic matter decomposition produced soluble organic compounds, however, not all of these organic compounds were further degraded by microorganisms and maybe DON was partly mineralized. Some previous research has showen that DON is composed of easily- and difficultly-degraded components (Zaman et al., 1999; Yu et al., 2002). Turnover time of the former was 1 to 5 days, while the latter was 80 days to 9 years. Results in this study also confirm that SON mineralization has a close relationship with an active part of DON. 5. Conclusions DOM contents in soils were significantly affected by fertility levels and land use patterns. DOC and DON contents in soils changed in
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