Changes in Phosphorus Fractions and Nitrogen

Journal of Integrative Agriculture© 2013, CAAS. All rights reserved. Published by Elsevier Ltd.Changes in Phosphorus Fractions and Nitrogen Forms During Composting of Pig Manure with Rice StrawLÜ Dui-an 1, 2, Y AN Bai-xing 1, W ANG Li-xia 1, DENG Zhi-qiang 1, 2 and ZHANG Y u-bin 31Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130102, P.R.China 2University of Chinese Academy of Sciences, Beijing 100049, P.R.China 3College of Plant Science, Jilin University, Changchun 130062, P.R.ChinaAbstractThe study was conducted to reveal P fractions and N forms changing characters during composting of pig manure with rice straw. During composting, the NH 4+-N concentration decreased and reached at a relatively low value (<400 mg kg -1) in the final compost, while the NO 3--N concentration increased. Total N losses mainly occurred during thermophilic phase due to the high temperature, the high NH 4+-N concentration and the increase of pH value. Labile inorganic P was dominated in the pig manure and initial compost mixture. During composting, the proportion of labile inorganic P of total extracted P decreased, while the proportion of Fe+Al-bound P, Ca+Mg-bound P and residual P increased. The evolutions of the proportion of labile inorganic P, Fe+Al-bound P and Ca+Mg-bound P were well correlated with the changes of pH value, organic matter and C/N ratio. Therefore, composting could increase the concentration of N and P and decrease the presence of NH 4+-N and labile P fractions which might cause environmental issues following land application.Key words : composting, nitrogen forms, phosphorus fractions, organic matter, pig manureINTRODUCTIONPig manure has sufficient amount of nutrients, especially N and P, which are the main limiting nutrients in majority of soil for crop production. However, long-term manure application to agriculture lands will lead to N and P accumulation in soils and an acceleration of soluble N and P transfer into water bodies via runoff (Huang et al. 2011; Li Q et al. 2012). To address this problem, strategies such as composting has been adopted in many countries. During composting, labile organic compounds are stabilized and converted into products that could be used as organic fertilizer and soil conditioner.The N forms are closely related to the degree of compost maturity (Bernal et al. 2009). Immature compost usually means high NH 4+-N concentration which can be easily lost through NH 3 volatilization following land application (Doydora et al. 2011; Vaddella et al . 2011). During composting, NH 4+-N can be immobilized or transformed to organic forms. Knowledge of evolution of N forms during composting is particularly important to assess agronomic and economic value of compost product, maturity of compost and risk following land application. The concentrations of P in animal manure and compost ranged from 1.5 to 39.3 g kg -1 (Sharpley and Moyer 2000). Most of the studies about P during composting only include total P (TP) content or the incrementReceived 25 October, 2012 Accepted 6 March, 2013LÜ Dui-an, Tel: +86-431-85542215, E-mail: lvduian_1147@; Correspondence WANG Li-xia, Tel: +86-431-85542215, E-mail: lxwang@1856LÜ Dui-an et al. between final and initial TP concentrations. Asknowledge of TP provides no information of the natureof chemical forms of P, fractionation of P should beintroduced in the study for it could provide an effectiveapproach for investigating availability, solubility andinter-conversion among P fractions (Shen et al. 2004).The P fractions in different environments such as insoils (Redel et al. 2008), sediments of marine, riversand lakes (Xiang and Zhou 2011) and sludge (Xieet al. 2011) have been widely researched, but littleinformation is available on the evolution of P fractionsduring composting.The objectives of current research were to determineP fractions and N forms evolution during compostingof pig manure with rice straw, and to investigate theparameters that influence P distribution. The changesof P fractions and N forms during composting wouldbe helpful to estimate the nutrient supply capacityof manure composting and the environmental riskfollowing land application.RESULTS AND DISCUSSIONCompost properties and humic substance contentTemperature is one of the most important parameters to monitor composting process (Fig. 1). During composting, microorganism decomposes organic matter (OM) and produces energy which increases the temperature. The temperature of the compost reached thermophilic phase in 1 wk and maintained for nearly2 wk, then decreased to ambient temperature ond 49, which meant the OM in the compost had been stabilized and the composting came to the end.OM losses can be defined as the percentage of decomposition (Fig. 2-A). The decomposition process mainly occurred during thermophilic phase of composting due to the high bacterial activities as a result of the high temperature and substantial biodegradable raw materials. The plateau value reached after 31 d of composting illustrated the lower bacterial activity due to lack of biodegradable compounds. This parameter is a good indicator of compost OM stability.The pH value increased throughout the composting because of the decomposition of acid-type compounds and the mineralization of organic compounds to inorganic compounds, such as NH4+-N (Fig. 2-B) (Paredes et al. 2000). The value of electrical conductivity (EC) increased at the beginning and thermophilic phase because of the mineralization of OM, but decreased at the latter phase which might be due to the precipitation of mineral salts, the volatilization of NH3and the stabilization of OM (Fig. 2-B). The C/N ratio of the study decreased, especially during thermophilic phase coinciding with the greatest OM degradation and then maintained at about 12 in the final compost (Fig. 2-C), indicating a good degree of maturity (Bernal et al. 1996). Dissolved organic carbon (DOC) decreased sharply at the beginning of the composting (Fig. 2-C), because these compounds were highly available to microorganisms. However, during thermophilic phase, DOC increased as a result of the continuously release of breakdown product through OM degradation. Later, DOC decreased again. Most of the DOC in the mature compost is present as humic substance (HS) and is unavailable for further decomposition (Hsu and Lo 1999).Temperature(°C)Composting time (d)Fig. 1 Compost and ambient temperature during composting of pig manure with rice straw.Changes in Phosphorus Fractions and Nitrogen Forms During Composting of Pig Manure with Rice Straw 1857Fig. 2 Changes in physicochemical properties of the compost mixture during composting of pig manure with rice straw. A, organic matter (OM) and total N (TN) losses. B, pH and EC. C, dissolved organic carbon (DOC) and C/N ratio. D, carbon of humic substance. E, urease activity and alkaline phosphatase (AlkP) activity. F, different N forms.FEDCBAE C (d S m -1)p HComposting time (d)2.02.53.03.54.04.55.0C /ND O C (g k g -1)Composting time (d)C a r b o n o f h u m i c s u b s t a n c e (g k g -1)Composting tme (d)A l k P a c t i v i t y (g p h e n o l k g -1 h -1)U r e a s e a c t i v i t y (g N H 4-N k g -1 d -1)Composting time (d)T N (g k g -1)I n o r g a n i c N (m g k g -1)Composting time (d)O M a n d T N l o s s e s (p e r c e n t a g e o f i n i t i a l v a l u e )Composting time (d)810121416182022242628303201234505101520253035404550HS comprises the most important fraction of OM for their unique properties, such as adsorption or reaction with metal ions, buffering pH, and as a potential source of nutrients for plants (Hsu and Lo1858 LÜ Dui-an et al.1999). HS can be separated into humic acid (HA) and fulvic acid (FA). The concentration of carbon of HS (C HS), carbon of HA (C HA) and carbon of FA (C FA) had a similar decreasing trend at the beginning of composting (Fig. 2-D), because of the degradation of simple organic carbon compounds (such as simple carbohydrates, fats and sugars) which could be easily mineralized and metabolized by microorganisms (Bernal et al. 2009). Then, these forms increased as a consequence of the fierce OM degradation during the following week. Later, C HS, C HA and C FA showed different evolution trends, C HA became stabled, while C HS and C FA decreased. The increase of C HA and decrease of C FA during composting represent the humification of OM and the maturity of the compost. Roletto et al. (1985) suggests that C HA/C FA>1 as the limited humification level for composts. The maximum value (2.53±0.10) of C HA/C FA observed in the final compost indicated that the composting was fully developed.Urease activity and alkaline phosphatase activityUrease is of agronomic value because the enzyme can catalyze hydrolysis of urea to CO2 and NH3. Urease is found widely distributed in nature such as soils, animals, plants and microorganism (Roscoe et al. 2000). At the beginning of composting, urease activity increased due to the high readily biodegradable OM and N-compound (Fig. 2-E), especially urea, which was rich in pig manure. Then, urease activity decreased and became stabled after 31 d of composting, which might be due to the stabilization of OM.Phosphatase is a key enzyme in the P cycle. Alka-line phosphatase (AlkP) activity decreased at the beginning of composting, and then increased to a plateau value on d 31 (Fig. 2-E), which differed from other studies. Ros et al. (2006) found AlkP activity increased at the beginning and reached maximum value in the mature compost, while Raut et al. (2008) found AlkP activity decreased after thermophilic phase of composting.Evolution of different N formsThe evolution of different N forms during composting is shown in Fig. 2-F. Total N (TN) increased slightly as a consequence of concentration effect due to the mass loss during composting (Bernal et al.2009). As NH4+-N could be easily transformed to organic forms under the metabolism of microorganisms or lost through NH3 volatilization, there was a little decrease of NH4+-N concentration at the beginning of composting. Then, an increase in the NH4+-N concentration was observed during the following week which might be due to the degradation of OM and N-compounds. Moreover, the high level of urease activity would accelerate the hydrolysis of urea and bring up the NH4+-N concentration. After 15 d of composting, the NH4+-N concentration decreased again, which could be explained by volatilization and immobilization processes. A high level of NH4+-N concentration indicates unstabilized materials during composting. The final NH4+-N concentration in the compost was below 400 mg kg-1, which was recommended as the maximum value in mature compost (Bernal et al. 1998). The NO3--N concentration was found at a low level at the beginning and mainly increased after thermophilic phase, because the high temperature and the high NH4+-N concentration inhibited the activity and growth of nitrifying bacteria (Morisaki et al. 1989). The decrease of NH4+-N and appearance of NO3--N are good indictors of the maturation process (Riffaldi et al. 1986). NH4+-N/NO3--N ratio decreased gradually during composting and reached near 2 in the final compost, which was still higher than the value (<0.16) established by Bernal et al. (1998) for mature compost, but lower than the result found by Huang et al. (2004). As different composting technologies and raw materials were used, the value varied at a wide range (Bernal et al. 1998; Huang et al. 2004; Amir et al. 2005; Haroun et al. 2009).TN losses in the final compost was (35.91±1.13)% of initial value (Fig. 2-A). At the beginning of composting, although there was a high level of NH4+-N concentration, the TN losses was low since the pH value was only near 7 which might limit NH3 volatilization. The greatest TN losses was observed during thermophilic phase due to the highChanges in Phosphorus Fractions and Nitrogen Forms During Composting of Pig Manure with Rice Straw 1859temperature, the high level of NH 4+-N concentration and the increase in pH value which would enhance the volatilization of NH 3 (Bernal et al . 2009). The plateau value of TN losses reached on d 31 meant that the OM and N-compound in the compost had become stabled. Low value of TN losses is desirable as it means the increases of agronomic and economic value in the compost product.Changes in P fractionsThe concentration and proportion of individual P fractions are shown in Fig. 3. The concentration ofall forms of P increased due to the decomposition of OM and the mass losses during composting. Pearson correlation matrices of P fractions with pH, OM, C/N ratio, C HS , DOC and AlkP activity were investigated to determine the influence of these selected parameters on the changes of P distribution during composting (Table 1).As the main fraction in the pig manure, labile inorganic P (P i ) accounted for (51.75±1.61)% of total extracted P. This compound is dominated by phosphate and is effective P fraction for plants uptake of P significantly (Turner and Leytem 2004). However, too much anthropogenic inputs of soluble102030405060708090100 Labile P iLabile P oFe+Al P Humic P Moderately labile P o P e r c e n t a g e o f t o t a l e x t r a c t e d P (%)Composting time (d)5 000DCBAT h e c o n c e n t r a t i o n o f P f r a c t i o n (m g k g -1)Composting time (d)5001 0001 5002 0002 5003 0003 5004 000T h e c o n c e n t r a t i o n o f m i c r o b i a l P (m g k g -1)Composting time (d)Microbial P49393123157105101520M i c r i o b i a l P (p e r c e n t a g e o f t o t a l e x t r a c t e d P )Composting time (d)493931231571010Fig. 3 Changes in P fractions during composting of pig manure with rice straw. A and B, the concentration of P fractions. C and D, the percentage of P fractions of total extracted P.1860LÜ Dui-an et al.P to soil will lead to excrescent labile P i transfer to water bodies which can contribute to eutrophication in freshwater ecosystems (Leytem and Bjorneberg 2009; McCallister et al. 2010). During composting, the largest decrease was found in labile P i. The proportion of total extracted P in this pool decreased from (52.52±0.13)% at the beginning to (27.45±0.35)% at the end of the composting. The decrease of the proportion of labile P i was significantly correlated with the decrease of OM (R=0.990**) and C/N ratio (R=0.997**) and the increase of pH value (R=-0.905***). As labile P i is mainly consisted of phosphate which could form insoluble precipitates as a result of sorption to clays, incorporation into interior structure of non-labile materials and reaction with metal ions (Forbes et al. 2005; Li H et al. 2012), the formation of HS an d the increase of pH value during OM degradation could accelerate the transformation of labile P i to more recalcitrant forms.Labile organic P (P o) is also considered to be soluble form and represents weakly absorbed P o in soils that includes DNA, phospholipids, and simple phosphate monoesters (Turner and Leytem 2004). These compounds have environmental significance even if present in relatively small concentration for their mobility (Stewart and Tiessen 1987). During composting, the proportion of labile P o decreased from (3.63±0.19) to (1.29±0.06)%. The decrease of labile P o was well correlated with the changes of C HS (R=0.778*) and DOC (R=0.873*). This might because labile P o was mainly contained in dissoluble organic matters.Microbial P represents P i and P o held in live mi-croorganisms. This compound is important for plants use, for it can turn over rapidly to labile P i and labile P o following land application of organic wastes (Forbes et al. 2005). The proportion of microbial P in the compost increased nearly throughout the composting and reached plateau value ((11.08±0.56)% of total extracted P) in the final compost. The increase of the microbial P was positively correlated with the decrease of OM (R=-0.896**) and C/N ratio (R=-0.824*) and the increase of pH value (R=0.975**). As found by Raut et al. (2008), there is an increase of microbial biomass during composting. The increase in microbial biomass might be responsible for the increase in the proportion of microbial P.The P extracted by NaOH and HCl can be consid-ered poorly soluble in the environment (Turner and Leytem 2004). These fractions include Fe+Al-bound P, humic P, Ca+Mg-bound P and moderately labile P o. During composting, the proportion of P extracted by NaOH and HCl increased, which meant the im-mobilization of P. Fe+Al-bound P only accounted for (8.38±0.17)% of total extracted P at the beginning of composting. However, this fraction increased and became quantitatively important in the final compost, accounting for (15.98±0.28)% of total extracted P. The largest increase was found in Ca+Mg-bound P. This form was recalcitrant and was dominated pool in the final compost, accounting for (40.93±0.34)% of total extracted P. On the contrary to labile P i, the in-creases of Fe+Al-bound P and Ca+Mg-bound P were well correlated with the decrease of OM (R=-0.955** and R=-0.956**, respectively) and C/N ratio (R= -0.943** and R=-0.980**, respectively) and the increase of pH value (R=0.893** and R=0.839*,respectively). Many studies have shown that phosphate would be adsorbed on the surface of Fe, Al, Ca, Mg and hydrox-ides (Danenlouwerse et al. 1993; Li H et al.2012). The large amounts of these ions in pig manure were responsible for the formation of Fe+Al-bound P and Ca+Mg-bound P (Turner and Leytem 2004). The hu-mic P and the moderately labile P o are the forms that may have environmental risk under the biological de-composition of microorganism. These two forms are mainly constituted of P o like P monoesters and phytic acid (Turner and Leytem 2004). The proportion of humic P of total extracted P decreased during com-posting, while the change of moderately labile P o wasTable 1Correlation coefficients of P fractions with selected physicochemical properties of the compost mixture during compos-ting of pig manure with rice strawP fraction pH OM C/N C HS DOC AlkP Labile P i-0.905** 0.990**0.997**0.2510.606-0.680 Labile P o-0.763*0.7510.7080.778*0.873*-0.123 Fe+Al-bound P0.893**-0.955**-0.943**-0.198-0.5590.740 Humic P-0.4940.7010.812*0.0220.262-0.236 Ca+Mg-bound P0.839*-0.956**-0.980**-0.289-0.5940.538 moderately labile P o-0.6780.7240.6590.3110.478-0.511 Residual P0.969**-0.937**-0.870*-0.546-0.828*0.638 Microbial P0.975**-0.896**-0.824*-0.465-0.797*0.718 *, ** statistically significant at the probability level 0.05 and 0.01 (2-tailed), respectively.Changes in Phosphorus Fractions and Nitrogen Forms During Composting of Pig Manure with Rice Straw 1861irregular.Although the change of the proportion of residual P was small, it showed statistically significant increase during composting. The increase of the proportion of residual P was well correlated with the decrease of OM (R=-0.937**) and C/N ratio (R=-0.870*) and the increase of pH (R=0.969**).Thus, the decrease order of P fraction at the be-ginning of composting was labile P i>Ca+Mg-bound P>Fe+Al-bound P>microbial P>residue P>humic P>moderately labile P o>labile P o, while in the ma-ture compost the order was Ca+Mg-bound P>labile P i>Fe+Al-bound P>microbial P>residue P>moderately labile P o>humic P>labile P o. The composting process allowed the transformation of labile P i and labile P o to more recalcitrant forms such as Fe+Al-bound P, Ca+Mg-bound P and residual P. This indicates that composting could be a useful way of managing ma-nure for P fixation and reducing agricultural P in run-off following land application.CONCLUSIONComposting could increase the concentration of N and P and decrease the presence of NH4+-N and soluble P fractions which might cause environmental problems following land application. The NH4+-N concentration was at a relatively low value (<400 mg kg-1) in the final compost. The NO3--N concentration mainly increased during mature phase. Composting allowed the transformation of labile P i which was dominated at the beginning of composting to more recalcitrant forms such as Ca+Mg-bound P which was dominated in the final compost, Fe+Al-bound P and residual P. The decrease of labile P i and the increase of Ca+Mg-bound P, Fe+Al-bound P, residual P and microbial P were well correlated with the increase of pH value, the decrease of OM and C/N ratio.MATERIALS AND METHODS Experimental procedureThe composting pile (about 400 kg) was prepared constituting of pig manure and rice straw in a ratio of 3:1 (w/w, fresh weight). The initial moisture content of the compost was adjusted to about 65% and maintained at the same level by adding necessary amount of water weekly. Composting was conducted in a vessel with a volume of 640 L (length×width×height=0.8 m×0.8 m×1.0 m). In the button of the vessel, there were eight PVC tubes with ventilation holes in sidewalls. Forced aeration was used during the composting and the air-blowing was controlled by a timer which was set at 30 min ventilation per hour. The aeration flow was controlled at 6 m3 h-1 by rotameters. Temperature was measured daily at the center of the pile by temperature sensors and the ambient temperature was achieved simultaneously.The composting was lasted for 49 d. And seven samples were taken during the experimental time (on d 1, 7, 15, 23, 31, 39, 49). Each sample was taken by mixing nine sub-samples together (three from the top, three from the middle and three from the button) and then divided into two parts: one was kept at 4°C and the other was air-dried, grounded and sieved to <0.25 mm for further analyses. The physicochemical properties of the experimental raw materials are shown in Table 2.Table 2 Physicochemical properties of the experimental raw materials (dry matter basis)Parameter Pig manure Rice straw pH 7.10±0.01 7.03±0.01 EC (dS m-1) 3.65±0.01 3.01±0.02 NH4+-N (mg kg-1)3454.70±83.30444.40±40.16 NO3--N (mg kg-1) 129.10±14.85 60.30±34.65 Organic matter (g kg-1)806.03±3.73886.86±9.92 Total N (g kg-1) 31.49±0.62 11.42±0.01 Total P (g kg-1) 15.80±0.23 3.63±0.10 Values are means±standard error.Chemical analysisThe compost samples were analyzed for pH and EC in 1:10 (w/v) water extracts (He et al. 2009). NH4+-N and NO3--N were extracted with 2 mol L-1 KCl from fresh samples and were then determined by the indophenol blue method and cadmium reduction method. The moisture contents of the samples were determined after dried at 105°C for 24 h. Ash content was determined by combustion at 550°C for 8 h. OM was determined by subtraction of ash content. Total organic carbon was calculated as 58% of OM (Zmora-Nahum et al.2005). TN was measured by Kjeldahl method (Bremner 1996). DOC, C HS extracted with 0.1 mol L-1NaOH, and C FA from the NaOH extraction at pH 1.0 were determined by potassium dichromate oxidic method (Pereira et al. 2006).C HA was calculated as the difference in C HS and C FA. Activities of urease and AlkP were measured according to Cang et al. (2009).Losses of OM and TN were calculated from the initial (A1) and final (A2) ash contents following the formula (Paredes1862LÜ Dui-an et al.et al. 2000):OM losses (%)=100-100×[A1(100-A2)]/[A2(100-A1)]TN losses (%)=100-100×[(A1N2)/(A2N1)]W h e r e N1a n d N2a r e t h e i n i t i a l a n d f i n a l T N concentration, respectively.Sequential P fractionationThe procedure of sequential P fractionation modified acco r ding to the method of Hedley et al. (1982) was shown in Fig. 4. The organic component of the HCl extracts (moderately labile P o) was added which was not included in the Hedley fractionation procedure, as this pool should not be ignored in compost. TP in the extracts was analyzed following sulfuric acid-potassium persulfate digestion in an autoclave while P i was analyzed directly. Residual P after the final extraction and TP in the compost samples were determined after digested with concentrated H2SO4 and H2O2. The P i and TP in the extracts and digest were all assessed by the ascorbic acid method after the specified treatment. The recoveries between the sum of individual P fractions and TP ranged from 86 to 93%. The differences were caused by the accumulation losses between each procedure.Fig. 4 Sequential P fractionation procedure. 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