Zhihong LU, Suxia JI, Meiliang ZHANG, Jingrong LIU, Jiegang ZHANG

1. College of Land Resources and Environment, JAU, Nangchang 330045, China;

2. SQM (Beijing) Trade Co., Ltd., Beijing 100022, China;

3. College of Agronomy, JAU, Key laboratory of Crop Physiology/Ecology and Genetic Breeding of Jiangxi Province, Nangchang 330045, China

Organic matter is an important part of soil fertility[1-2], and a kind of repository of nitrogen,phosphorus, sulphur and trace elements. These nutrients usually undergo from organic form into mineralization to maintain crop growth. What’s more, organic matter increases exchange capacity of cation, conductive to soil fertility maintenance[3-8].Besides,as a major source of activity of trace elements, organic matter enhances maintenance capacity of soil moisture and drought tolerance capacity of crops[9].It is widely known that components’ activities of soil organic matter tend to be volatile. Yuan[10]proposed the coefficient of oxidation stability according to labile organic matter,that is,the readily oxidizable-to-the difficulty oxidizable rate (kos) reflected quality of soil organic matter, which has been closely linked to oxidation resistance of humus, affecting soil fertility. Humus, the core fraction of soil organic matter, is an organic compound synthesized by organic residues entering soils through microbial action, including humin,humic acid (HA)and fulvic acid(FA).Specifically,HA plays key inenhancing soil absorbing capacity,helping the soil retain moisture, and encouraging the formation of good soil structure,and FA is simple in structure and strong in acidity that diffuse easily.Therefore, HA/FA is always used to indicate soil ripening, and the higher HA/FA,the higher soil ripening degree and the better soil organic matter[11].Many researches available[12-14]have demonstrated that the application of organic fertilizers increases content of organic matter and improves the quality, because organic matter in soils would be updated by the newly applied ones and total humus quantity and organic-mineral complex would keep growing accordingly. The research explored the effects of fertilization treatments on organic matter content and components of red paddy soils by location test for a long term, which is of great importance for scientific improvement of soil fertility.

Materials and Methods

Test materials

The long-term located fertilization was started from 1981 in a test site of Jiangxi Agricultural University on basis of waterloggogenic paddy soil developed from red clay of the Quaternary period. The test area was 1 m2, and soil fertility kept moderate and higher.It is heavy loam with pH of 6.5, containing 32.560 g/kg organic matter,1.789% total N, 90.0 mg/kg available nitrogen, 20.8 mg/kg available P and 87.0 mg/kg available K.

The early-season rice cultivars included Guangluai No.4,73-07,Weihua No.49, Chaofengzao, 92-48, 85-01 zhongyou 402, Nongda No.228,Bo65, Jinyou No.160, and Zaoxian No.52, and late-season rice included 0021, Jingnuo No.6, Xianyou No.6,N120, Guangnong No.16, Jinyou No.220 and Wanxian No.2000.

Test design

The test designed 5 treatments,as follows: T1 without fertilizers, T2 applied with single chemical fertilizer,T3 applied with milk vetch (base fertilizers for early-season rice) (similarly hereinafter) and chemical fertilizers,T4 applied with milk vetch, straws(base fertilizer for late-season rice)and chemical fertilizer, and T5 applied with milk vetch,pig manure(base fertilizer for late-season rice) and chemical fertilizer. Every treatment was with three replications, as per randomized block design.

Except of T1,quantities of N,P2O5and K2O kept the same of the rest treatments. Specifically, the quantities of N, P2O5and K2O were 225, 112.5 and 225 kg/hm2, respectively during 1981-1986, 240, 120 and 240 kg/hm2during 1987-1991, 270, 135 and 270 kg/hm2in 1992-2003, and 300, 150 and 300 kg/hm2in 2003-2007.In general, fertilizers were divided into two halves for early and late-season rice.Specifically,chemical fertilizers included urea, calcium-magnesia phosphate fertilizer, and potassium chloride; organic fertilizers included milk vetch,straws of early rice, and pig manure.Milk vetch and fresh straws were applied at 22 500-30 000 kg/hm2and dry straws at 3 000 kg/hm2; pig manures were applied at 15 000 kg/hm2;nutrient contents of organic fertilizers were computed as per measured results. It is concluded that proportions of organic nitrogen and inorganic nitrogen were 0∶10, 4∶6, 5∶5 and 6∶4 in T2, T3, T4 and T5 respectively. It is notable that insufficient of P and K in organic fertilizer should be made up by chemical fertilizer. P fertilizers were applied as base fertilizers for early or late-season rice; N and K fertilizers were divided into base fertilizer, tillering fertilizer and granule fertilizer applied at the proportion of 5∶2∶3.

Table 1 Contents of soil organic matter g/kg

Sampling method

Genetic characters of early and late-season rice were explored and computed.Soil samples of late-season rice were collected from soil horizons of 0-20 cm, and ground after drying.The agrochemical traits were measured with sifters(1 and 0.25 mm).

Measurement method

Readily oxidizable organic matter was measured as per the method of Yuan’s[15].Additionally,sodium pyrophosphate was extracted by potassium dichromate method; soil organic matter, total N, available P, available N, available K and pH value were measured by agricultural chemistry analysis[16].

Data processing

The test data were processed by Excel and DPS.

Results and Analysis

Effects of long-term located fertilization on organic matter of paddy fields

As shown in Table 1, soil fertility improvement tended to be volatile upon fertilization treatments. Generally speaking, averaged contents of soil organic matter were T5, T4, T3, T2,and T1. Of T1, organic matter kept decreasing. For example, averaged decreases were 1.77% during 1981-1992,and 1.05%during 1997-2007.It can be concluded that the decreases changed from fast to slowly, possibly caused by that rice used readily oxidizable parts firstly. Of T2, the change proved in contrary to that of T1.For instance, averaged decrease was just 0.39% in 1981 -1992 and 1.89% in 1997 -2007. Of T3, organic matter content maintained unchanged basically, but kept declining upon time,which incorporated that the application of just fresh milk vetch at 22 500 kg/hm2can not increase organic matter. As for T4 and T5, the contents of organic matter grew significantly. For example, the growths reached 0.62%and 0.70%in 1981-1997,but dropped in 1997-2007.According to analysis of variance, the application time of organic and inorganic fertilizers is a significant factor influencing organic mat-ter content of soils and fertilization treatment is an extremely significant factor. Specifically, T1 and T2 showed significant differences in terms of organic matter content; organic matter contents of T3,T4 and T5 were of extremely significant differences with that of T1; the contents of T4 and T5 showed extremely significant differences with that of T2; T3 and T5 showed significant differences in organic matter content;the organic matter content of T4 was of insignificant differences with those of T3 and T5.These have demonstrated that with nutrient quantities equivalent, longterm application of organic and inorganic fertilizers maintained and even improved content of organic matter compared with application of just chemical fertilizer.

It can be concluded from Table 1 that content of soil organic matter would achieve balance gradually by long-term application with a fertilizer and fluctuated a little, because soil clays are limited in protecting organic matter[17].However, balance points are different upon fertilization treatments.In the research, the treatment where early-season rice was applied with milk vetch and chemical fertilizer and late-season rice was applied with pig manure and chemical fertilizer (T5)performed the best in increasing organic matter,followed by the treatment where early-season rice was applied with milk vetch and chemical fertilizer and late-season rice applied with straw and chemical fertilizer(T4).

Table 2 Dynamic changes of humus components by long-term fertilization

Table 3 The percentages of readily oxidizable organic matter in total organic matter %

Dynamic changes of humus components by long-term located fertilization

As shown in Table 2, humic acid and HA were decreasing in T1 and FA kept declining in 1987-2002, but grew from 2007. In general, averaged decreases of humic acid reached 0.23 g/kg in 1987 -1992, 0.086 g/kg in 1992-1997, 0.48 g/kg in 1997-2002 and 0.029 g/kg in 2002 -2007. Besides, humic acid, HA, and FA changed similarly in the treatment with single chemical fertilizer and the treatment with organic/inorganic fertilizers,but humic acid, HA, and FA always kept higher than those of T1 and T2.These demonstrated that the consumption of organic matter in soils can not be well supplemented only relying on root stubbles if fertilizers are not applied or just chemical fertilizers are supplied. Consequently, humic acid with high transformation capacity still keeps in short, so that it is harder for soils to supply nutrients and soil fertility declines. Long-term application of organic and inorganic fertilizers is conductive to retain soil fertility,but it is also crucial to adjust other factors because organic matter transformation is under influence of many aspects.

It can be concluded from Table 2 that the proportion of humic acid declined significantly, the proportion of HA in humic acid grew a little, and HA/FA increased a little in T1. In T2,however, the proportion of humic acid dropped remarkably, the proportion of HA in humic acid changed from increasing to decreasing, and HA/FA increased in former 10 years and kept decreasing thereafter. In T3, T4 and T5, the proportion of humic acid increased significantly, and the proportion of HA dropped a little in humic acid.As for HA/FA,it almost kept higher in the treatments than that of T2 in the same year, with an exception of 1992, and higher than that of T1 in1987 and 1997,which coincided with the research made by Shen[19].In other years, nevertheless, HA/FA always maintained lower than that of T1, and decreased in T3, T4 and T5. It is believed that the transformation capacity and activity of humus have significant effects on soil fertility, of which HA plays a key role in soil structure formation and nutrient supplement and adjustment. Still, some believed that it is HA that plays a key role in accumulation and updating of organic matter,for it is a component characterized by molecular weight,high activity and degree of oxidation. The research indicated that application of organic and inorganic fertilizers is conductive to increases of HA and FA, but HA/FAwould go down. Besides, the function of FA in soil material movement, plant nutrient and soil fertility should be further explored, which is in consistent with the research conducted by Dou et al.[20].

Effects of long-term located fertilization on content of readily oxidizable organic matter

Shen[19]conducted measurements on readily oxidizable organic matters of soils in growth stages of early and late-season rice and the results demonstrated that activities of organic fertilizer, soil trace element and soil enzyme grew significantly in T3, T4 and T5, advancing decomposition of organic matter. Hence, content of readily oxidizable organic matters kept higher than those of T1 and T2 during growth stages of early and late -season rice. The research measured and analyzed readily oxidizable organic matters and the proportions in organic matter during 1987-2002.As shown in Fig.2, long-term application of organic and inorganic fertilizers would improve content of readily oxidizable organic matters. Specifically, the content of readily oxidizable organic matters tended to be volatile in T1, but generally kept decreasing, possibly caused by fallen leaves and root stubbles. In T2,the content was significantly higher than that of T1,but lower than those of T3, T4 and T5, which has proven that application of organic and inorganic fertilizers would enhance content of readily oxidizable organic matters.From Table 3,it is known that the proportion of readily oxidizable organic matters kept lower in T1,and the quality of organic matter was poor; the proportion of T2 was higher than that of T1, but lower than those of T3, T4 and T5, which indicated that mixed application of organic and inorganic fertilizers perform better in terms of improving quality.According to the research conducted by Wang et al.[21],fresh organic matter enhanced the content of humic acid in humus, improving quality of organic matter. Besides, the content of organic matter always maintained higher in T5 than that of T4 in 22 years, but rice yield kept lower in 16 years. These incorporated that T4 performed better in improving organic matter in soils compared with T5,and it is important for increasing rice yield to improve soil organic matter.

Table 4 Correlation of organic matter stabilization with rice yield and soil nutrient

The relationship of oxidation stability with soil nutrient and yield

As shown in Table 4, soil organic matter showed extremely positive correlation with readily oxidizable organic matters and significant correlation with total nitrogen and hydrolyzable nitrogen contents and rice yield.Besides, the content of readily oxidizable organic matters was of significantly positive correlation with total nitrogen, available P and yield. Specifically, the correlation (r=0.818 1*) of rice yield and readily oxidizable organic matters was higher than the correlation (r=0.802 0) with organic matter,which suggested that it is of significance for improving quality of organic matter to increase content of readily oxidizable organic matters.

It can be concluded that positive correlation was shown of total nitrogen with organic matter, readily oxidizable organic matters, hydrolyzable nitrogen,and available P.Furthermore,the significant correlation of hydrolyzable nitrogen with organic matter and yield has demonstrated that some hydrolyzable nitrogen of high availability exists in soils, and content of organic matter is key for supplying nitrogen of soils.

Available P of soil was of positive correlation with readily oxidizable organic matters (r=0.839 2*),total nitrogen (r=0.858 0*), hydrolyzable nitrogen(r=0.811 1*),total P(r=0.919 0*),which indicated that readily oxidizable part of organic matter is an important source of available P and total P, hydrolyzable N and total N showed close relationship with available P. Besides,higher correlation coefficient of available P and rice yield (r=0.946 3*)proves the importance of available P content for improving rice yield.

With a low proportion in plough layer, organic matter plays a key role in soil fertility, for it is an important source of crop nutrients,improving soil biological characters. Therefore, content and quality of organic matter are crucial indices for evaluating soil fertility.In the research, readily oxidizable organic matter was decreasing in the treatment with just chemical fertilizer,suggesting that chemical fertilizer is limited in improving organic matter. In contrast, the treatment with organic and inorganic fertilizer improved quality and content of organic matter.

Conclusion

The research proved that the dynamics of soil organic matter of the different fertilization treatments showed significant differences, In the premise of equal nutrient(nitrogen and phosphorus and potassium), combining application of organic-inorganic was benefited for the accumulation of organic matter in paddy soil than without fertilizer treatment or chemical fertilizer treatment; the dynamics of soil humic acid, HA and FA of chemical fertilizer only and combining application of organic-inorganic treatments had basically the same trend, But the contents of humic acid, HA and FA of combining application of organic-inorganic treatments had been higher than that without fertilizer and chemical fertilizer treatment. Moreover combining application of organic-inorganic treatments was benefited for improving the contents of HA and FA,but decreasing HA/FA ratio as an extension of time.Combining application of organic-inorganic was benefited for improving the contents of readily oxidizable O.M;And the contents of soil organic matter in long-term experiment and the contents of readily oxidizable O.M were highly significant positive correlation and the contents of soil total nitrogen,avail nitrogen and potassium were significant positive correlation; the contents of soil readily oxidizable O.M and the contents of soil total nitrogen,avail P and rice yield were significant positive correlation. thereinto, the correlation (r=0.818 1*)between the rice yield and soil readily oxidizable O.M was higher than the correlation (r=0.802 0)between the rice yield and soil organic matter. It showed the soil readily oxidized organic matter had greater contribution to the rice yield.

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