Based on agricultural nitrogen (N) balance model and field experiments, the impacts of farming system changes of Taihu Region of China on surface water environment were studied. During past 60 years, farming systems changed greatly in Taihu Region. The traditional method of manure collection and application was replaced by chemical fertilizer utilization gradually. Chemical N fertilization intensity decreased greatly due to the abolition of “3 crops per year” and reduction of cropland area in 1990-2010. Crops depleting soil fertility increased, while those improving soil fertility decreased, leading to an excessive dependence on chemical fertilizer application, which increased the risks of soil N loss to surface water environment in Taihu region. However, field experiments showed that the agricultural N loss with runoff only accounted for 2% of fertilizer N application rate. The majority of N was exported by crop harvesting. Our findings showed that the agricultural N loss might not be the main source of N pollution in Lake Tai after 2000. To control N pollution of Lake Tai, more attention should be paid to industrial and domestic wastewater from urban and rural areas, wastes from livestock and poultry breeding, bait input for aquaculture, etc in the Taihu Region, China.
Land Use and Land Cover Change (LUCC) is the core of global environmental change researches. According to the UN Agenda 21 in 1992, LUCC research will be the focus in the 21 century ([
Nutrient cycles link agricultural systems to their societies and surroundings. The inputs of nutrients (mainly N and P) are essential for high crop yields [
Cropping system is one of the most important factors influencing agricultural nutrient cycle [
The Taihu region (119˚13'E - 121˚19'E, 30˚46'N - 32˚14'N) is situated at the heart of Yangtze River Delta. It covers an area of 1.76 × 104 km2, accounting for 47.7% of the total area of Taihu Basin. Four districts (Suzhou, Wuxi, Changzhou and Zhenjiang) and two counties (Danyang and Jurong County of Zhenjiang district) are included in this region (
Low-lying areas from east to west, consists of plain, hilly mountains and water respectively occupying 58.3%, 14.2% and 17.5% of Taihu region area. It belongs to the north subtropics monsoon climate, and is very vulnerable to extreme climate and season fluctuation [
annual mean precipitation is 1100 - 1400 mm, with rains being unevenly distributed throughout the year. The wet season (May-September) accounts for 60.5% of total rain in a year. Water and heat over the same period and abundant heat resources are beneficial to crop growth in subtropical areas. Paddy rice is the dominant crop, and the other major crops are wheat, rapeseed, and soybeans. Cropping systems is two crops per year with rice- wheat being most popular in this region.
Agricultural nitrogen budgets were calculated based on agricultural nitrogen balance model according to Roy [
Nsurp is defined as the ratio of yearly N surplus and crop acreage in Taihu region. Ninput is nitrogen input from outer sources, including fertilizer application (Nfe), manure application (Nma), atmospheric nitrogen deposition (Ndep) and nitrogen fixation (Nfix); Noutput is nitrogen output from agricultural system, including N export by harvesting (Nhav), N loss to atmosphere through ammonia volatilization and denitrification (Ngas). All terms are in unit of kg N ha−1 yr−1.
Input: Nitrogenous fertilizer use and manure N input
Nfe is defined as the ratio of annual N fertilizer utilized to cropping land. Data of all N fertilizer forms were converted into element N to estimate N fertilizer input. Nma is calculated as animal manure N input divided by crop acreage of Taihu rigion. Manure N applied (Nma) in 1990s was calculated as 1/9 of Nfe [
Input: Atmospheric N deposition
Ndep is atmospheric N deposition including dry and wet deposition of nitrogen. According to [
Input: symbiotic N fixation
Nfix is defined as the ratio of symbiotic N fixation in paddy field and crop acreage of Taihu region. Symbiotic N fixation by microorganisms in paddy field was obtained by multiplying rice planting area and N fixation ratio of rice which is set to 45 kg N ha−1 according to [
Output:
Nhav is defined as the ratio of yearly harvested N and crop acreage of Taihu region. It is obtained as follows.
Y is crop yield of Changzhou, Suzhou and Wuxi County. i = 1, 2. 1 represent rice, 2 represent wheat. r is crop N absorb ratio in growing season, it is set to 0.019 and 0.03 ([
Ngas is defined as the ratio of N export to atmosphere through ammonia volatilization and denitrification and cropland acreage of Taihu region. Ngas is set to 45% of N fertilizer application respectively according to [
Data of N fertilizer intensity, crop planting area, crop yield, arable land area, land area, population size, GDP in 1985-2010 were obtained from Statistical Yearbooks of Changzhou, Suzhou, Wuxi and Zhenjiang ([
Data of the above parameters in 1949-1984 were obtained from Wuxi County [
Data of agricultural N loss through runoff was obtained from field experiment carried out in a typical cropland of the Taihu Region in 2012-2013.
The field experiment was established at the Changshu Agroecological Experimental Station, Institute of Soil Science, Chinese Academy of Sciences (31˚33'N, 120˚42'E) in Nov 2012-Nov 2013 (
The experiment utilized one field with an area of 15 by 10 m. The field was divided into 2 sample plots, with 3 replications per plot. One represents farming systems in 1980s. The other represents 2000s. The experiment was started in Nov. 2012, the beginning of the wheat season, and continued for one integrated rice-wheat rotation. In the 2000s sampling plot, chemical fertilizers including 50% urea, 30% ammonium carbonate and 30% compound fertilizer were applied at rates of 300 kg N ha−1 in the rice season and 250 kg N ha−1 in the wheat season. In the 1980s sampling plot, 70% ammonium carbonate and 30% organic fertilizer (pig manure) were applied at rates of 188 kg N ha−1 in the rice season and 157 kg N ha−1 in the wheat season. For N application, 30% was basally applied, 40% was topdressed at the tillering stage, and the remaining 30% was topdressed at the ear differentiation stage for each crop. 6 pots were installed near the experimental plots to collect runoff from the field. By doing this, the results of this experiment were expected to appropriately reflect the conventional farming systems in 1980s and 2000s in the Taihu Lake Region. Total nitrogen concentration of runoff samples were analyzed by an ultraviolet spectrophotometer.
As is shown in
The ratio of food crops and cash crops was 9:1 in 1949-1978. Cash crops include green manure crops, vegetables, rapes, potatoes, melons, beans, etc. Among the cash crops, green manure crops were conductive to maintain soil fertility and to guarantee grain yield due to strong ability of symbiotic N fixation. Therefore, the planting area of green manure crops expanded significantly year by year, occupying 35.2% - 69.7% of total in 1949- 1978. After 1978, the ratio of food crops and cash crops decreased gradually. The ratio was 8:2, 7:3 and 6:4 in 1980s, 1990s and 2000s, respectively (
Fertilization intensity in this study refers to the number of N fertilizer utilized to cropland in unit of kg N ha−1. As shown in
Periods | Cropping system | Food crops/Cash crops | Fertilization Intensity | Fertilization Structure | |
---|---|---|---|---|---|
Organic/Inorganic fertilizer ratio | N/P/K | ||||
1949-1978 | 2 to 3 | 9:1 | 98-above 700 kg N ha−1 a−1 | 9:1 | From 1:0:0 to 1:0.29:0.09 |
1979-1989 | 3 to 2 | 8:2 | 738 - 437 kg N ha−1 a−1 | 3:7 | 1:0.23:0.005 |
1990-now | 2 | from 7:3 to 6:4 | 578 - 445 kg N ha−1 a−1 | 1:9 to 0:1 | 1:0.08:0.13 |
year” [
Fertilization structure refers to the ratio of organic fertilizer and chemical fertilizer or the proportion of net nitrogen, phosphorus and Potassium in chemical fertilizer applied to cropland. Organic fertilizer includes manure from livestock, poultry and human being, waterlogged compost, green manure, methane fermentations waste etc. Chemical fertilizer includes ammonium bicarbonate, urea, compound fertilizer, ammonia water, phosphorus fertilizer and potassium fertilizer etc. In 1949-1978, organic fertilizer was collected and applied to cropland from many sources such as straw compost, human waste, rubbish, mud mixed compost and biogas fertilizer etc. It occupied about 90% of agricultural area of Taihu region in 1949-1978. After the 1978 reform, a large number of farmers moved to township enterprises to work. Moreover, it’s hard and insanitary to collect and ret organic manure. Therefore, organic fertilizer was replaced by chemical fertilizer gradually. The ratio of organic fertilizer and chemical fertilizer was 3:7, 1:9 and zero in 1980s, 1990s and 2000s. Chemical N fertilizer was received adequate attention while P and K fertilizer were lack of attention during 1949-1978. N and P were excessive and K was lack in soil during 1979-1990. With more and more attention paid to water environment caused by excess N input, P and K fertilizer application have been gradually appreciated recently. Chemical N, P and K fertilizer accounted for about 84.6%, 6.0% and 9.4% of the total chemical fertilizer respectively.
Changes of agricultural N input, output and surplus in the Taihu region since 1949 were shown in
Manure N has ever been the biggest contributor of agricultural N in 1949-1978. But the contribution decreased after 1978 until organic fertilizer was fully replaced by chemical N fertilizer in 2000 (
Runoff samples were collected for 7 times separately from 1980s and 2000s sample plots. 3 times were in the wheat season and 4 times in the rice season. Results of agricultural N loss with runoff for the 1980s and 2000s sample plots are shown in
Period | 1949-1978 kg N ha−1 | 1979-1989 kg N ha−1 | 1990-1999 kg N ha−1 | 2000-2010 kg N ha−1 | |
---|---|---|---|---|---|
Input (A) | Nfe | 67.8 (23.4) | 333.9 (59.1) | 492.5 (79) | 374.3 (86.1) |
Nma | 169.2 (58.4) | 178.0 (31.5) | 73.2 (11.8) | 0 (0) | |
Ndep | 7.6 (2.6) | 7.6 (1.3) | 22.8 (3.7) | 38 (8.7) | |
Nfix | 45 (15.5) | 45 (8) | 33.5 (5.4) | 22.4 (5.2) | |
Subtotal | 290 (100) | 564 (100) | 622 (100) | 435 (100) | |
Output (B) | Nharv | 88.7 (71.6) | 142.3 (45) | 157 (38) | 120 (38.1) |
Ngas | 30.5 (24.6) | 150.2 (47.6) | 221.6 (53.6) | 168.4 (53.5) | |
Nloss | 4.7 (3.8) | 23.4 (7.4) | 34.5 (8.3) | 26.2 (8.3) | |
Subtotal | 123.9 (100) | 315.9 (100) | 413 (100) | 314.6 (100) | |
Balance (A-B) Nsurp | 165.7 | 245.8 | 208.9 | 120.1 |
Contribution percentage shown in parentheses.
Season | Sampling Date | A | B | C = A*B/1600 |
---|---|---|---|---|
TN Concentration | Runoff | N Loss through runoff | ||
mg/L | L | kg/ha | ||
1980s wheat season | 2012.11.23 | 24.7 | 141.9 | 2.2 |
2012.12.30 | 19.9 | 141.0 | 1.8 | |
2013.02.26 | 24.3 | 138.6 | 2.1 | |
sum | 6.1 | |||
1980s rice season | 2013/7/6 | 2.3 | 176.0 | 0.3 |
2013/7/21 | 3.8 | 125.0 | 0.3 | |
2013/8/19 | 3.1 | 143.0 | 0.3 | |
2013/8/29 | 2.9 | 162.0 | 0.3 | |
sum | 1.2 | |||
2000s wheat season | 2012.11.23 | 27.0 | 141.9 | 2.4 |
2012.12.30 | 21.6 | 142.6 | 1.9 | |
2013.02.26 | 50.5 | 142.6 | 4.5 | |
sum | 8.8 | |||
2000s rice season | 2013/7/6 | 2.7 | 176.0 | 0.3 |
2013/7/21 | 4.2 | 166.0 | 0.4 | |
2013/8/19 | 17.1 | 166.0 | 1.8 | |
2013/8/29 | 7.5 | 168.0 | 0.8 | |
sum | 3.3 |
plot, the N export was 157 kg N ha−1 in wheat season and 176 kg N ha−1 in rice season, accounting for about 62.8% and 58.7% of fertilizer N input, respectively. Field experiments showed that N loss through runoff plays a minor role in agricultural N balance and crop harvesting is the main pathway of N export in both 1980s and 2000s.
With the process of urbanization in the Taihu region, large quantity of cropland was turned into construction land [
Green manure crops have ever playing an important role in cash crops in Taihu region. It is found that planting green manure crops such as Astragalus sinicus L can not only replenish nutrients to agricultural system, but also reduce environmental pollution ([
With the decrease of cropland area and the understanding of agricultural N pollution, less chemical fertilizer was applied to croplands in 2000s. Present fertilization intensity (rice season: 300 kg N ha−1; wheat season: 250 kg N ha−1) are far exceeding the optimum demands of crops for nitrogen (rice season: 150 kg N ha−1; wheat season: 225 kg N ha−1) in Taihu Region ([
Agricultural N balance in Taihu region has been in surplus since 1949 (
In late 1978, the implementation of rural household contract system mobilized farmers’ enthusiasm for production. More chemical N fertilizer was applied to croplands. Although the fertilization intensity decreased, chemical N fertilizer loss increased because of its soluble characteristics, causing negative effect on surface water environment. Water quality of Lake Tai deteriorated from Grade III in 1980s to Grade IV in 1990s. While in 2000s, chemical N fertilization intensity decreased significantly, the water quality of Lake Tai became worse to Grade V. It indicates that N pollutant sources of Lake Tai may have changed since 2000.
There are many other pollutant sources of Lake Tai, such as industrial and domestic waste water discharge from urban and rural areas, pollutant from intensive feeding of livestock and poultry. While strict measures were implemented to reduce urban and industrial wastewater discharge to Lake Tai after 1998. More than one thousand heavy pollution enterprises in Taihu region were forced to reach drainage standards before discharge. But the Water quality of Lake Tai still deteriorated in 2000s. On one hand, it is probably because the total discharge of urban and industrial wastewater increased along with population growth and economic development. Comparing with 1990, the total population in Taihu Region in 2012 increased 180 million, and the GDP increased 50 times. On the other hand, wastewater discharge from livestock and poultry feeding, excretion from human and animals, and bait input for aquaculture in rural areas increased N pollution in Lake Tai. Along with the improvement of people’s living standard in Taihu region, the demands for meat and eggs increased sharply, which promoted the development of intensive livestock and poultry feeding. But only a small amount of manure was processed into organic fertilizer and applied to croplands. A large quantity of manure was stacking randomly which is easily to loss with rainstorm runoff. By using the method of N isotope, Xing et al. [
Calculated agricultural N surplus decreased significantly from 1990s to 2000s, while the water quality of Lake Tai deteriorated year by year. Field experiments showed that agricultural N loss with runoff was very low, being only about 2% of fertilizer N application rate. A large quantity of N was exported by crop harvesting. It indicated that agricultural N loss might not be the main source of N pollution in Lake Tai during 1990s-2000s. It was suggested that in highly industrialized and urbanized areas, the N pollutant sources such as industrial and domestic wastewater from urban and rural areas, wastes from livestock and poultry breeding, bait input for aquaculture, etc. should be paid more attention.
This work was funded by the Natural Science Foundation of China (41030745, 41201496), the Natural Science Foundation of Jiangsu Province (BK20141513), the Knowledge Innovation Program of the Chinese Academy of Sciences (KZZD-EW-10-04) and Key “135” Project of Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences (NIGLAS2012135005). The authors greatly appreciated the time and efforts given by anonymous reviewers and by the editors in evaluating our manuscript.