(4) Increasing stability of regional agriculture industry
4. Evaluation of environmental influence of the ryegrass winter cropping
To make high efficiency of fertilizer in agricultural practices to reduce the disturbance of the overflowed nutrients upon natural ecosystems, particularly the natural water environment, is an important target of the ecological fertilization. The market of inorganic fertilizers has been dramatically expanded with the development of agricultural industry (Zhu, 1990).
According to the statistic data, the domestic market of fertilizer in China has increased from 25.903 million tons in 1990 to 44.116 million tons in 2003, and the market of nitrogen fertilizer raised from 16.384 million tons in 1990 to 21.499 million tons in 2003 (China Statistical Yearbook, 2004).
Although the increased application of fertilizer in agriculture has enhanced the productivity of crops, the efficiency of the fertilizing is fairly low due to inappropriate application (in terms of both amount and methods) of fertilizer. Based on the survey in various locations in China, the availabilities of the fertilizers in China are as low as 30-35%
for N fertilizers, 10-20% for P fertilizers, and 35-50% for K fertilizers (Sun et al., 2001). The low fertilizer utilizations have brought on a huge input of the nutrients into natural environment via pathways like runoff, leaching, denitrification, adsorption and erosion.
Statistics showed that the national average of fertilizer leaching and runoff amounts to 40%
of the total fertilizers application (Wang, 2001). Excessive use of fertilizer has caused various contaminations in over 1/5 of arable lands in China (Zhao, 1994), led to huge economic loss, and tipped off the fragile balance in agricultural ecosystems (Qu, 1999). Non-point source pollution by agricultural practice and fertilizer application has become an important research topic in the international ecological community.
As the IRR system has gradually gained its popularity, the studies on its environmental influence, especially on N, P cycles through the system, become eminently important. The boosting up of the nutrients utilization in the IRR system via the improved nutrient cycle in the rice-ryegrass-soil system will assuredly declare the establishment of the ecological fertilization embodying in the IRR system.
4.1 Effects of agricultural nitrogen and phosphorus on pollution and eutrophication of water bodies
Non-point source pollution stands in the contrary site against point source pollution when viewed with the angle of in type. It refers to the larger scope, results from the forces of rainfall-runoff, the dissolved or solid pollutants go through from non-specific locations into adjacent receiving water bodies (such as rivers, streams, lakes, artificial reservoirs and gulf).
Besides, when the pollutants are excessive accumulated in water, eutrophication would occur.
Agricultural non-point source pollution, owing to the broad range it affects, the great amount of pollutants output, and the strong effect to the adjacent water bodies, has been considered to be the major source of water pollution.
Negative effects of agricultural activities on surface and ground water quality have been a topic of concern in many parts of the world for several decades. The agricultural pollutants come mainly from the improper utilization of fertilizer and pesticide, especially the huge amount of nitrogen and phosphate fertilizer. The loss of N, P fertilizer in agriculture is mainly driven by the process of precipitation-runoff process, what is more, the main body of the loss happens during the storm runoff event. (Beegle et al., 2000; Behrendt and Opitz, 2000).
Moreover, the inappropriate land utilization and improper farmland management cause the erosion, will accelerate loss of N, P in the soil (Mander, 2000). An increased nutrient loss from arable land due to the nutrient surplus in agricultural systems is suggested as the leading reason for the water quality deterioration of the lake in China (Fan et al., 1997; Jin et al.1999).
The consequences following the great amount loss of agricultural N, P are the increasingly severe water pollution problems (Wang, 2001). Data has shown that most of the
lakes in China have undergone the change from oligotrophi state to the eutrophication state since the twentieth century (Zhang et al., 2002; Lu et al., 2002). For example, for Tai Lake and Dianchi Lake, the agricultural non-point source pollutants has composed of 57, 38 and16% of the water pollutants NH4+-N, TP and COD, respectively (Wu, 2001).
Wang et al. (1996) found that NH4+
-N in paddy soil is more easily absorbed by soil particles than NO3
--N, and the NH4+
-N could be hold in soil more steadily, while the NO3
--N could easily filtrate into the beneath of profile, rendering that the NO3
--N is easily entering groundwater via leaching. Wang et al. (1997) have also proved that the major form of N leaching in paddy soil is NO3
--N.
4.2 Nitrogen and phosphorus application when the ryegrass winter cropping in paddy
Surface runoff, leaching drainage, subsurface runoff are the major pathways of soil phosphorus loss. Since it is putative that P harbors a higher affinity with soil, it is recognized that there are few conditions when the element P moves as downward vertical migration (Heckrath et al., 1995; Sims et al., 1998). Soil corruptions caused by surface runoff are thought to be the chief pathway through which P flows from soil to accepting water body.
In fact, the transfer of P in interflow in soil could happen in areas where are overdue-fertilized. Zhang et al. (2001) concluded in a simulated paddy assay that the concentration of P transferred in interflow in paddy soil could be higher than previously realized, leading the soil P to leach into groundwater.
In the IRR system, fertilization is performed in two stages including the IRG growing season in winter and the paddy rice growing season from spring to summer. IRG is much responsive to fertilizer and fertilizer tolerable, so that heavy fertilization is easily practiced for harvesting more green fodder. Fortunately, the IRG has high ability to accumulate nitrogen, and the concentration of N could be as high as 3.5% in its aboveground (Yang et al., 1994). It was found that N input (31.8 g m-2) throughout fertilization was 28% less than the N output (59.8 g m-2) from harvesting green fodder, while a positive balance (+22.0%) in soil P was observed (Xin et al., 1998b). As above-mentioned, fertilizing in the IRG growing season is much safe for environment protection, and part of the nutrients fixed in the IRG tissues will release into soil with a slow-release like process after the tissues were incorporated into the paddy soil. Associating with the soil improvement in many aspects in the IRR system as above mentioned, it is believed that the fertilizers applied in the rice growing season, which may highly risks the environment pollution due to the runoff and leaching of the applied inorganic nutrients in the rainy season, could be reduced after a long term practice under the IRR system.
4.3 Control of outflows of nitrogen and phosphorus from the soil in the IRR system
Soil left bare after tillage operations is prone to erosion, and induces the run-off and leaching of nitrogen, phosphorus and pesticides into surface and ground water (Jürg Hiltbrunner et al., 2007). According to Xin et al. (2000), when the application of compound fertilizer goes to 750-1500kg hm-2, the productivity of the IRG increases in a dosage dependent manner, which may cause the oversupply of the nutrients, particularly N and P, and harm soil and surrounding environments. Hence, it is count for much to assess the environment influence of the practice of the IRR system.
Li et al. (2005) investigated the environmental burden under the fertilizing level of 375 and 750kg hm-2 for IRG winter cropping. The result showed that most of the N and P in soil were absorbed and fixed by the IRG tissues. Because the contents of N and P and the yield of the IRG were increased accompanying with the increase of the fertilizing rate, the volume of the fixed N and P by the IRG was directly proportional to the fertilizing rate. In the winter idled paddy, the N accumulated in weeds was 3.289 g m-2, while in the IRG cropped paddies received each 375 and 750 kg hm-2 inorganic fertilizer (N:P:K=15:15:15), the N fixed in the IRG tissues were as high as 9.120 and 12.920 g m-2, respectively. The N losses through infiltration in the idled paddy were 2.70 g m-2, even significantly higher than those in the IRG cropped fields (1.37 and 1.69 g m-2 under the 375 and 750 kg hm-2 fertilizing, respectively).
The N fixed by the weeds or the IRG were 1.2, 6.7 and 7.6 folds of those leached N in the idled paddy and the two IRG copped paddies, respectively (Li, 2005). Comparing to the N, much more effective P fixation was observed. It was found that the P fixed by the weeds or the IRG in the idled paddy and the two IRG copped paddies were 40, 144 and 198 folds of those leached (Li, 2005).
The N and P losses through infiltration leaching take up quite small proportion of the total N and P in soil and the IRG plant, and it was found that the losses were negatively proportional to the fertilizing rate, indicating that the promoted IRG growth by the fertilizing are favorable to fix the N and P in the soil-plant system. After rainy season came, the loss of NO3-N, NH4+
-N and P increased as the fertilizing rates rose, while the proportions of the lost N and P to the total N and P in soil and Plant were decreased as the fertilizing rates rose. The loss of N was mainly in form NO3-N, being consistent with the previous studies (Wang et al., 1996; Wang et al., 1997).
According to Li (2005), although no fertilizer was applied in the winter idled paddy, the losses of NO3
--N, NH4+
-N and P through soil infiltration were found to still be 26.18, 0.83, and 0.028 kg hm-2, respectively. The NO3
--N, NH4+
-N and P losses in the IRG winter cropped paddy received 375 kg hm-2 inorganic fertilizer (N:P:K=15:15:15) were 13.24, 0.43 and 0.021 kg hm-2, significantly lower than those in the winter idled paddy. Similarly, when the fertilizing rate increased to 750 kg hm-1, N loss in case the IRG cropped (NO3
--N 16.18 kg hm-2; NH4+
-N 0.70 kg hm-2) was significantly lower than that under fallow, while the P loss (0.029 kg hm-2) slightly increased for 4.4%, but the difference was not significant (Li, 2005).
These results revealed that the IRG coverage in the fallow season of rice can reduce the losses of N and P, which is helpful to cut down the influence of agricultural pollutants to surface and ground waters. Kurtz et al., (1946; 1952) indicated that the continuous soil cover
provided by the living mulch/cover crops is a good strategy for reducing soil erosion.
Macdonald et al. (2005) also reported that reductions in N leaching exceeded 90% when comparing cereal cropping systems with and without cover crops. In fact, among the various practices that reduce soil erosion and leaching of such nutrients as NO3--N, winter cover cropping has gained more acceptance by growers as it has been proved to be more effective in reducing soil erosion, N leaching and contamination of water (Meisinger et al. 1991;
Francis et al. 1994; Thorup-Kristensen et al., 2003).
It is obvious that the IRR system is an environment-friendly agricultural approach in the utilization of inorganic fertilizers. The safe nutrients supply in winter lets the IRG has chance to fix the N, P and K and other nutrients in its tissues, and then leaves in paddy soil in form of residues. The IRG residues release the nutrients fixed in their growing season in winter when they decompose gradually, and thus the releasing process is slower, so that more nutrients can be effectively absorbed by the subsequent rice. On the other hand, the IRG provides soil cover and nutrients carrier in winter, which greatly decreased the runoff and leaching of the soil N and P. Therefore, the addition of the IRG in the traditional rice production in southern China, which provides land cover and carrier or filter of soil nutrients, constituted the environment-friendly mechanism of the IRR system as a typical style of the ecological fertilization. The practice is considered to be contributive to the protections of surface and ground waters.