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Agriculture, Ecosystems and Environment 138 (2010) 44–50

Contents lists available at ScienceDirect

Agriculture, Ecosystems and Environment

journa l homepage: www.e lsev ier .com/ locate /agee

ffects of long-term fertilization on corn productivity and itsustainability in an Ultisol of southern China

han Huanga, Weijian Zhanga,b,∗, Xichu Yuc, Qianru Huangc

Institute of Applied Ecology, Nanjing Agricultural University, Nanjing 210095, ChinaKey Laboratory of Crop Ecology, Physiology and Production, Ministry of Agriculture, Institute of Crop Science,hinese Academy of Agricultural Sciences, Beijing 100081, ChinaJiangxi Institute of Red Soil, Jinxian 331700, China

r t i c l e i n f o

rticle history:eceived 29 November 2009eceived in revised form 18 March 2010ccepted 18 March 2010

eywords:orn (Zea mays L.)ong-term fertilizationoil qualityltisols

a b s t r a c t

Appropriate fertilization practices play a critical role in enhancing crop yields, as well as in achievingsustainable increases in crop production through improving soil quality. In the present study, we exam-ined the effects of long-term fertilization (started in 1986) on crop yield and its sustainability under adouble corn (Zea mays L.) cropping system in an Ultisol of southern China. Results showed that althoughcorn yields were higher in the fertilized treatments compared to the unfertilized control, inorganic fer-tilizer application alone (N and NPK) resulted in declining trends in both corn yields and fertilizationeffects (defined as the yield difference between the fertilized treatment and the control), especially inthe N treatment. In contrast, significant increasing trends in both corn yields and fertilization effectswere observed in the manure-applied treatments. Manure amendments significantly increased the con-tents of soil organic carbon and total N and the availability of soil P and K, while long-term inorganic

ield trendsfertilization alone accelerated soil acidification, especially in the N treatment. In addition, there was nosignificant correlation between the yield trend and climatic factors (weather trends) over the period ofstudy. Thus, it is likely that the long-term sustainability of corn production under manure application isattributed mainly to the improved soil quality. Our results also suggest that manure amendments, partic-ularly in combination with inorganic NPK, should be the recommended fertilization practice to enhance

soil q

corn yields and improve

. Introduction

Corn (Zea mays L.) is an important staple crop for food, live-tock feed, and biofuels at global scale (Cassman et al., 2003;andis et al., 2008; Edgerton, 2009). In China, corn accountedor 57% of the total consumption of feed grain in 2007 (Yin,009). With economic growth and the improvement in living stan-ards, increasing consumption of livestock products will lead toigher feed grain demand (Delgado, 2003). Therefore, future cornequirements are expected to increase rapidly in China (Wangt al., 2009). Although corn production has increased dramati-ally in the last three decades in China, these highly intensive

orn cropping systems have led to detrimental environmentalmpacts that in the long term may threaten their sustainabil-ty as a result of reduced soil quality (Liu and Diamond, 2005).fforts to combine the continuous increase in corn produc-

∗ Corresponding author at: Institute of Crop Science, Chinese Academy of Agricul-ural Sciences, Beijing 100081, China. Tel.: +86 10 62156856; fax: +86 10 62156856.

E-mail addresses: zhangweij@caas.net.cn, nolarui@126.com (W. Zhang).

167-8809/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.agee.2010.03.015

uality in the Ultisol.© 2010 Elsevier B.V. All rights reserved.

tion and maintain soil quality are therefore critical in ensuringfuture food security in China (Jin et al., 2005; Wang et al.,2009).

In 2007, the sown area and yield of corn in North and North-east China accounted for 66% and 68%, respectively, of the totalcorn area and production in China (National Bureau of Statistics,2008). However, due to the rapid increase in pig production andpork consumption, there is a great shortage of corn for feed grainin southern China (Yin, 2009). The wide geographical separationbetween the main areas of corn production and corn consump-tion has caused serious economic and environmental problems inChina, while, the intensive corn cropping systems have inducedsignificant soil degradation, groundwater pollution, and green-house gas emissions in North and Northeast China (Huang andSun, 2006; Ju et al., 2006; Li et al., 2010). These negative effectswill aggravate as the requirements for corn increase (Tao et al.,

2003; Xiong et al., 2010). On the other hand, there is 15.2 millionha of upland in southern China, comprising 40% of the total crop-land in this area (Huang, 1995). In addition, the climatic resourcesin southern China are favorable for corn cropping with abun-dant rainfall and high temperatures (Wang et al., 2009). Recently,

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S. Huang et al. / Agriculture, Ecosyst

eld experiments have demonstrated that high and sustainablerop yields could be achieved in a wheat–corn rotation systemn the uplands of southern China (Zhang et al., 2009b). Thus,ncreasing attention has been paid to corn production in theseplands, but recommended management practices for these sys-ems have not been formulated (Huang et al., 2009; Zhang et al.,009b).

In southern China, soils are mainly Ultisols and Oxisols, char-cterized by low productivity and high risk of erosion resultingrom intensive weathering aggravated by inappropriate utiliza-ion (Zhang and Xu, 2005). Thus, sound fertilizer management,articularly organic manure application, could restore crop pro-uctivity and its sustainability through improving the quality ofhe degraded soils (Xu et al., 2003; Huang et al., 2006; Hao et al.,008; Bi et al., 2009). For example, several studies have documentedhat manure application led to increased soil organic carbon (SOC)ontent and nutrients, improved soil porosity and structure, andncreased soil microbial activity (Li and Zhang, 2007; Zhong andai, 2007; Hao et al., 2008). However, previous studies dealt mainlyith rice fields in southern China. Limited information is avail-

ble on the long-term impact of fertilization management on cornroductivity and its sustainability in upland soils (Zhang et al.,009b).

Results from long-term field experiments provide opportunitieso explore the mechanisms underlying the effects of fertilizationn crop yield and soil quality (Richter et al., 2007). Therefore,e conducted field monitoring and multi-year data analyses inlong-term fertilization experiment under a double corn crop-

ing system in an Ultisol of southern China (started in 1986). Ourbjectives were: (1) to determine the effects of long-term fertil-zation on corn yields and soil quality; and (2) to investigate the

echanism underlying any trends in corn yields and fertilizationffects.

. Materials and methods

.1. Site description

The long-term field experiment was initiated in 1986 underdouble corn cropping system at the Institute of Red Soil, Jinx-

an County (28◦37′N, 116◦26′ E, 26 m above sea level), Jiangxirovince, China. This site is under a typical subtropical climate,

ith two distinct growing seasons, wet (March–June) and moder-

tely dry (July–September). Mean annual temperature and rainfallre 17.2 ◦C and 1549 mm, respectively. Detailed means and rangesf monthly rainfall, maximum and minimum temperatures arehown in Table 1. The soil is developed from quaternary red clay,

able 1eans and ranges of monthly rainfall, maximum and minimum temperatures from 1981

Month Rainfall (mm) Tmax (

Mean Range Mean

January 81.1 20.9–279.7 8.9February 107.4 17.7–227.1 11.1March 171.6 83.3–390.2 15.0April 218.8 75.0–392.0 21.7May 222.4 52.9–365.7 26.8June 294.1 140.2–680.4 29.6July 139.3 29.5–456.5 33.6August 130.0 23.4–422.1 32.9September 72.3 1.3–216.9 28.9October 58.8 0.8–185.2 24.0November 76.1 1.7–292.1 18.0December 41.4 0.0–150.7 11.9

max and Tmin represent mean monthly maximum and minimum temperatures, respectiv

nd Environment 138 (2010) 44–50 45

with a clay (<0.001 mm) content of 260.0 g kg−1 in the surfacesoil (0–15 cm).

2.2. Experimental design

The experiment was a randomized complete block design withthree replicates (plot size 22.2 m2). Five treatments were selectedfor the present investigation: (1) control (unfertilized); (2) inor-ganic nitrogen fertilization (N); (3) balanced fertilization withinorganic nitrogen, phosphorus, and potassium (NPK); (4) manureapplication (M); and (5) inorganic NPK application combined withmanure (NPK + M). The application rates of inorganic fertilizerswere 60 kg N ha−1, 13 kg P ha−1, and 33 kg K ha−1 for each corn-growing season. The inorganic N, P, and K fertilizers used wereurea, calcium superphosphate, and potassium chloride, respec-tively. Manure in the form of pig manure was applied at a rate of15 Mg ha−1 on a fresh weight basis with the water content of 70% foreach growing season. Based on dry weight, the manure contained28.3 g N kg−1, 10.3 g P kg−1, 9.8 g K kg−1. The fertilization rates wererecommended by the Jiangxi Institute of Red Soil, which were basedon soil tests at the beginning of the experiment. All the P fertilizerand manure were applied as basal fertilizers before soil ploughingin each corn season, while half of the total N and K fertilizers wereincorporated as basal fertilizers with the other half applied at the10 leaf stage.

The first and second corn crops were sown in early Apriland late July, and harvested in late July and early November,respectively. Corn was seeded by hand in hills at intervals of50 cm in lines and 33 cm in rows. Herbicides and pesticides wereapplied during the growth period when needed. Corn cultivarswere changed every 5 years. All aboveground crop biomass wasremoved from the plot following corn harvest, simulating thatcorn residues were used mainly as fuels by local farmers. Grainyields were determined by harvesting the whole area of eachplot.

2.3. Soil sampling and analysis

Soil samples were collected by the core method after harvest ofthe second corn crop in November 2007. In each plot, six cores weretaken to a depth of 15 cm and were mixed as a composite sample.The fresh soil samples were air-dried and then passed through a

2-mm sieve for analysis.

Soil organic carbon (SOC), total N (TN), pH, total P (TP), totalK (TK), available P (AP), and available K (AK) in soil samples weredetermined. The analytical methods were: pH (1:1, soil/H2O), TP(molybdenum method), TK (flame photometer method), AP (Olsen

to 2007.

◦C) Tmin (◦C)

Range Mean Range

6.2–13.2 3.0 −0.2–5.35.8–16.7 5.1 2.2–9.0

10.5–17.7 8.6 6.0–11.119.7–26.3 14.6 12.8–17.524.6–30.0 19.6 17.8–21.427.5–31.3 22.9 21.4–24.430.7–35.9 26.0 24.5–27.430.6–34.8 25.7 24.5–27.126.5–31.5 22.0 20.1–24.020.5–26.0 16.6 13.9–19.514.0–21.1 10.6 8.5–13.3

8.2–14.5 4.8 2.5–7.4

ely.

46 S. Huang et al. / Agriculture, Ecosystems and Environment 138 (2010) 44–50

Table 2Mean yields (1986–2008) and their time trends as affected by long-term annual fertilization.

Treatments First corn Second corn

Mean yield (Mg ha−1) Yield trend (Mg ha−1 year−1) R2 Mean yield (Mg ha−1) Yield trend (Mg ha−1 year−1) R2

Control 0.74 d −0.024* 0.19 1.01 c −0.020 0.11N 2.22 c −0.099*** 0.45 1.31 c −0.046* 0.18NPK 3.73 b −0.039 0.08 3.43 b −0.008 0.00M 3.26 b 0.121** 0.36 2.93 b 0.103*** 0.47NPK + M 5.26 a 0.081* 0.18 4.63 a 0.091* 0.23

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ifferent lowercase letters within the same column indicate significant differences* Yield trends significant at P < 0.05.

** Yield trends significant at P < 0.01.*** Yield trends significant at P < 0.001.

ethod), AK (1 mol L−1 ammonium acetate extraction, flame pho-ometer method). All methods are described in detail by Lu (2000).n 1986, SOC was determined using a wet oxidation method with2Cr2O7 and concentrated H2SO4. Total N was measured using aicro-Kjeldahl digestion method. In 2008, SOC and TN were mea-

ured by dry combustion with an elemental analyzer (Elementar,ario Max, Germany). Carbonates were not present in the samples.comparative measurement showed that no significant difference

n SOC content was observed between the wet oxidation methodnd the elemental analyzer method, while TN content was signif-cantly higher by 8% with the elemental analyzer than with theigestion method. Therefore, soil TN content in 1986 was correctedor comparison.

.4. Data analysis

Fertilization effects were examined first in terms of the long-erm yield means, and then the annual relative yield values weresed to investigate the time trends. The relative yield was calcu-

ated as the yield difference between the fertilized treatment andhe control, thus eliminating the effects of year-to-year variationsn trend analyses due to changes in weather and cultivars (Bi et al.,009).

To avoid the confounding effect of crop yield response to theumulative effects of annual fertilizer application, we used theield in the control treatment to evaluate the relationship betweenorn yields and growing-season weather parameters by pairwiseorrelation analyses. In addition, to separate the direct effect of aeather parameter on crop yield, partial-correlation analyses wereerformed to eliminate the confounding effect of other weatherarameters (Peng et al., 2004).

Analysis of variance (ANOVA) was performed to determine theffects of long-term fertilization on corn yields and soil proper-ies. Means were tested using the Least Significant Difference (LSD)ith a significance level of P < 0.05. Linear regression analyses were

mployed to test the significance of time trends in actual andelative corn yields and weather parameters at P < 0.05. All statis-ical analyses were performed using SPSS software 11.0 (SPSS Inc.,hicago, IL, USA).

. Results

.1. Corn yield and its trend

For the first corn crop, application of inorganic fertilizer (Nnd NPK) and/or organic manure (M and NPK + M) significantlyncreased mean grain yields compared to the unfertilized control

Table 2). Grain yields of the second corn crop displayed similaresponses to fertilization, except the N treatment, which showed noignificant difference relative to the control. Although NPK yieldsere numerically greater than those of the M treatment, differ-

nces were not statistically significant; but the combined treatment

g treatments (P < 0.05).

(NPK + M) gave significantly much higher yields in both croppingseasons.

Grain yields of the first corn crop showed significant decliningtime trends in the control and N treatments, whereas manure appli-cation resulted in significant increasing trends (Table 2). Like thoseof the first crop, grain yields of the second corn crop showed sig-nificant increasing trends in the M and NPK + M treatments, andsignificant decreasing trends in the N treatment and in the con-trol, but the latter was not statistically significant. Grain yields inthe NPK treatment showed decreasing, but not significant trendsin both the first and second corn crops.

3.2. Trends in fertilization effects

Fertilization effects (i.e., in terms of relative yield) showed sig-nificant increasing trends in the M and NPK + M treatments in boththe first and second corn crops, while no significant change wasobserved in the NPK treatment (Fig. 1). Fertilization effects in theN treatment declined in both cropping seasons, but were not sta-tistically significant for the second corn crop.

3.3. Trends in weather parameters

Warming trends in mean maximum and minimum tempera-tures were observed during both the first and second corn-growingperiods, though the increasing trend of mean maximum tempera-ture in the first corn-cropping season was not significant (Fig. 2).Rainfall displayed no significant change during either the first orsecond corn-growing period (Fig. 3).

3.4. Correlations between corn yield in the control treatment andweather parameters

Pairwise correlation analyses indicated that grain yields of thefirst corn crop in the control treatment were significantly andnegatively correlated with minimum temperature, whereas no sig-nificant relationships were found between yields of the second cornand any weather parameter (Table 3). In both, the first and secondcorn-growing seasons, rainfall was significantly negatively corre-lated with maximum temperature, which in turn was positivelycorrelated with minimum temperature.

Partial-correlation analyses showed that grain yields in the con-trol treatment were not significantly correlated with any weatherparameter for either the first or second corn crop due to the signif-icant correlation between maximum temperature and minimumtemperature (Table 4).

3.5. Long-term fertilization effects on soil properties

Soil pH was significantly lower in the N treatment than inthe control, while showing slight increases in the plots receiving

S. Huang et al. / Agriculture, Ecosystems and Environment 138 (2010) 44–50 47

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ig. 1. Trends in relative yield from 1986 to 2008. Relative yield is the yield differen* and *** indicate relative yield trends significant at P < 0.01 and P < 0.001, respectiv

anure (M and NPK + M) (Table 5). Long-term (since 1986) contin-ous corn cropping led to a slight decline in the content of SOC inhe control. The contents of SOC and total N (TN) did not signif-

cantly differ among the control, N, and NPK treatments, whereasong-term manure application significantly increased their SOC andN contents compared to the control, similar to the content of totaland the availability of P and K. Although total K content did not

ary among various treatments, long-term corn cropping resulted

tween the value from the fertilized treatment and that from the control treatment.

in dramatic reductions in the content of available K in the controland N treatments.

4. Discussion

Long-term trends in crop yield may arise from the combinedeffects of changes in climate, cultivars, soil, and management prac-tices (Kucharik and Serbin, 2008). Our results indicated that corn

48 S. Huang et al. / Agriculture, Ecosystems and Environment 138 (2010) 44–50

Fig. 2. Mean daily maximum (a) and minimum (b) temperatures during the corn-growing period. * and ** indicate temperature trends significant at P < 0.05 and P < 0.01,respectively.

Fig. 3. Total growth-period rainfall during the first (a) and second (b) corn-growing periods.

Table 3Pairwise correlations between corn yield in the control treatment and weather parameters during growth periods.

First corn Second corn

Yield Rainfall Tmax Tmin Yield Rainfall Tmax Tmin

Yield 1 0.01 −0.33 −0.49* 1 0.03 −0.24 −0.29Rainfall 1 −0.59** −0.37 1 −0.43* −0.05Tmax 1 0.86*** 1 0.72***

Tmin 1 1

Tmax and Tmin represent mean maximum and minimum temperatures during the corn-growing period, respectively.* Correlation coefficients significant at P < 0.05.

** Correlation coefficients significant at P <0.01.*** Correlation coefficients significant at P < 0.001.

Table 4Partial-correlations between corn yield in the control treatment and weather parameters during growth periods.

First corn Second corn

Yield Rainfall Tmax Tmin Yield Rainfall Tmax Tmin

Yield 1 −0.12 0.10 −0.37 1 −0.01 −0.05 −0.15Rainfall 1 −0.55 0.26 1 −0.57 0.41Tmax 1 0.83** 1 0.76**

Tmin 1 1

Tmax and Tmin represent mean maximum and minimum temperatures during the corn-growing period, respectively.** Correlation coefficients significant at P < 0.01.

Table 5Topsoil properties (0–15 cm) as affected by long-term fertilization.

Treatments pH SOC (g kg−1) TN (g kg−1) TP (g kg−1) TK (g kg−1) AP (mg kg−1) AK (mg kg−1)

Initial (1986) 6.0 9.4 1.06 0.62 11.36 12.9 102.0Control 5.9 a 8.6 c 1.05 b 0.68 d 11.17 a 11.8 d 70.0 cN 5.2 b 9.5 c 1.14 b 0.76 cd 11.08 a 10.7 d 62.7 cNPK 5.7 ab 9.6 c 1.11 b 0.83 c 11.46 a 21.8 c 158.3 bM 6.4 a 11.7 b 1.32 a 1.04 b 10.91 a 51.4 b 163.3 bNPK + M 6.2 a 13.7 a 1.43 a 1.47 a 11.59 a 68.0 a 211.7 a

Different lowercase letters within the same column indicate significant differences among treatments (P < 0.05). SOC, soil organic carbon; TN, total nitrogen; TP, totalphosphorus; TK, total potassium; AP, available phosphorus; AK, available potassium.

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S. Huang et al. / Agriculture, Ecosyst

ield trends in the Ultisol were not associated with any changesn climatic factors (weather trends) over the period of study. Bi etl. (2009) also reported that although significant warming trendsere observed in the region, there was no significant correlation

etween temperature and rice yields, but fertilization had signif-cant effects on both actual yield and relative yield trends. Ouresults showed that total rainfall during the crop-growing seasonas not significantly associated with corn yields, which may beue to the large interannual and monthly variation of rainfall andhe uneven distribution between the first and second crop seasonsTable 1). Because the optimal amount of crop water demand iselative stable, both drought and waterlogging are adverse to croproduction, resulting in the correlation between rainfall and cornields not statistically significant in the long time series (Tao etl., 2006). Moreover, the differential trends in corn yields amongarious fertilization treatments in the present study (Table 2) sug-est that the effect of cultivar changes on yield trends is limitedelative to that of fertilization, which is consistent with Fan et al.2008). Therefore, we speculate that the observed yield trends orig-nate mainly from fertilizer application and its consequent impactn soil quality (Saleque et al., 2004; Richter et al., 2007).

Unbalanced inorganic fertilizer application (N) increased cornields in the short term, but negatively affected soil fertility in theong term (Table 5), thus decreasing the sustainability of corn yieldsTable 2) and the fertilization effect (Fig. 1). Given the low con-ents of plant-available P and K, it is likely that the supply of theseoil available nutrients in the unbalanced fertilization treatmentsould not meet the rapid uptake for crop growth, thus constrain-ng the increase of corn yields (Edmeades, 2003). Our results showhat long-term N (urea in this study) application alone significantlyeduced soil pH, which agrees with the report by Zhong and Cai2007). Recent evidence also indicates that urea addition is effectiven stimulating nitrification in subtropical soils, thereby resulting inignificant soil acidification (Zhao and Xing, 2009). Since Ultisols inouthern China are inherently acid due to heavy weathering (Xu etl., 2003), leaching of soil nutrients such as Ca and Mg will increases a consequence of increasing acidity (Sarno et al., 2004; Noble etl., 2008). Moreover, further acidification may promote the mobil-ty and release of active Al with the associated risk of Al toxicityMcGahan et al., 2003). In a recent study conducted on the same soilype, Zhang et al. (2009a) showed that the contents of exchangeablea2+ and Mg2+ were significantly reduced in the long-term inor-anic fertilization treatments compared to the initial levels, whilehe exchangeable Al3+ and H+ were significantly increased. Thus,nsufficient supplies of soil available nutrients and accelerated soilcidification due to long-term unbalanced inorganic fertilizer appli-ation are the most likely causes of the significant negative trendsn corn yields and fertilization effects.

Although balanced inorganic fertilizer application (NPK) couldead to higher mean corn yields, both the actual and relative yieldshowed slightly decreasing trends in the long term (except the rel-tive yield in the second corn crop) (Table 2 and Fig. 1). Thus, iteems that balanced NPK application could only sustain the currentorn yields in this Ultisol. Strong soil acidification and unfavorablehysical structures are two main limiting factors for crop produc-ion with high yields in the Ultisol (Xu et al., 2003; Tejada andonzalez, 2007). Zhang et al. (2009b) reported that while the bal-nced fertilization of NPK could increase crop yields, such practiceas not likely to be able to maintain high grain yields over the long

erm in the Ultisol, which is probably due to soil acidification andegradation of soil structure resulting from continuous inorganic

ertilization without organic inputs. Results from the same experi-

ental site also showed that no improvement in soil structure andggregate stability was observed in the inorganic fertilizer-appliedlots as compared to the control (Li, 2009). Previous studies haveemonstrated that increasing organic matter inputs was a key mea-

nd Environment 138 (2010) 44–50 49

sure to improve soil structure and increase nutrient contents, andthus enhance crop yield in Ultisols (Sarno et al., 2004; Zhang andXu, 2005). Inorganic fertilization alone without recycling of cropresidues in the present study was insufficient to provide enoughorganic matter inputs to increase SOC that plays a critical role inalleviating soil acidification and improving soil structure in theUltisol (Xu et al., 2003).

Manure application alone or combined with inorganic NPKresulted in significantly higher corn yields and in sustainable yieldgrowth in the Ultisol (Table 2 and Fig. 1). Long-term manure appli-cation led to significantly higher values of SOC and TN contents andto higher availability of soil P and K, relative to unmanured plots(Table 5). Recently, Zhang et al. (2009b) also reported that cropyields were significantly positively correlated with SOC, TN andavailable P in subtropical Ultisols. In addition, amendment withorganic manure could improve the micronutrient status in soils,and thus increase crop production (Li et al., 2007). Moreover, resultsfrom the same field trial showed that long-term manure applica-tion promoted the formation of soil macroaggregates and increasedaggregate stability (Li, 2009). The improvement in soil aggrega-tion and structure could benefit crop growth through stimulatingroot distribution and uptake of water and nutrients (Pachepsky andRawls, 2003; Bronick and Lal, 2005). Furthermore, the increase inaggregate stability is likely to enhance soil resistance to erosionthat has become a major threat to crop production and agriculturalsustainability in southern China (Zhang et al., 2004; Zhang and Xu,2005). Since the manure used in the field experiment was frompig farms, the current animal and crop production systems werenot linked. However, given the strengths of manure application forimproving crop yield and soil fertility, a recoupling of livestock andcrop production systems is needed (Naylor et al., 2005). On theother hand, manure amendments may simultaneously cause neg-ative effects on natural environment, such as nutrient losses andgreenhouse gas emissions (Ju et al., 2006; Li et al., 2010), whilethe present study focused only on the yield sustainability. Thus, anintegrated assessment on the overall sustainability of the doublecorn cropping system under manure application in the Ultisol ofsouthern China is needed.

5. Conclusions

Significant differences in corn yield and its sustainability werefound among fertilization regimes in an Ultisol of southern China.Unbalanced inorganic fertilizer application (N) not only couldnot sustain corn yields in the long term, but also caused serioussoil acidification. Although balanced inorganic fertilization (NPK)resulted in significantly higher grain yields for both first and sec-ond corn crops, it could only sustain the current corn yields. Manureapplication alone or combined with NPK resulted in significantlyhigher corn yields, and in sustained growth of corn yields in the longterm. Significant warming trends were observed at the experimen-tal site, but no significant correlations existed between temperatureand corn yields. Thus, we conclude that the positive trends incorn yields and fertilization effects in the manure treatments areattributed mainly to the improvement in soil quality, as expressedin the increase in the contents of SOC and TN and the availabilityof soil P and K. In addition, we suggest that manure amendments,especially combined with inorganic NPK, should be encouraged toincrease corn yields and improve soil quality in the Ultisol.

Acknowledgements

This work was supported by the National Basic Research Pro-gram of China (2009CB118601), the Program for New CenturyExcellent Talents in University (NCET-05-0492), and the National

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ey Technology R&D Program of China (2006BAD02A15). We thankwo anonymous reviewers and the editor for their constructiveomments on the manuscript.

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