1.3 Nitrogen processes in the vegetable production system
1.3.2 Comparison of nitrogen output in vegetable production systems with that in
10
concentration of the irrigation water determines the nitrogen contribution by irrigating. A nitrogen balance study carried out in Japan showed 30%-43% nitrogen contribution from irrigation water due to the high nitrogen content (4-12 mg/L) in underground water (Kyaw et al., 2005). Shallow groundwater used as an irrigation source in China’s north plain also has high nitrate concentrations resulting in a high nitrogen supply. Less nitrogen is taken into vegetable production system via irrigation in China’s Tailake region because river water is used instead of shallow groundwater.
1.3.2 Comparison of nitrogen output in vegetable production systems with that in
11
than that in conventional crop soil (Wang et al., 2002). Cao et al. (2008) evaluates that there is more than half of the nitrogen input from fertilizer temporarily left in the vegetable soil profile.
Soil ammonia nitrogen content increased 21 times and nitrate nitrogen content increased 22 times after soil planting patterns changed from paddy to vegetable (Yin et al., 2004). Nitrate nitrogen is the predominant residual nitrogen in soil, which is 5.9-6.2 times higher than ammonia nitrogen in vegetable soil (Wang et al., 2002). Most nitrate nitrogen is concentrated in the upper soil layer than in the rest of the profiles because of the nitrification of ammonium nitrogen and mineralization of organic matter occurring in the aerobic upper soil layer (Byrnes, 1990). Remarked residual nitrate nitrogen occurred with content reaching 936 mg/kg in the top 0.1 m of soil, and then the catch crop significantly reduces the average soil nitrate nitrogen to 195 mg/kg during the fallow period (Shi et al., 2009). In addition, nitrate nitrogen in soil easily moves continuously downwards with the free flow of water in the soil profile as opposed to ammonium nitrogen. Ju et al. (2006) found the amount of residual nitrate nitrogen in the 0-90 cm soil layer was 270-5038 kg N/ha in vegetable soil and 172-1452 kg N/ha in maize-wheat soil. The corresponding range of values in the 90-180 cm soil layer are 224-3273 kg N/ha in vegetable soil and 96-1993 kg N/ha in maize-wheat soil. The large amounts of residual nitrate nitrogen in the 90-180 cm soil layer indicates a substantial leaching potential of nitrate in the vegetable production system.
1.3.2.3 Nitrogen loss
Nitrogen loss is closely correlated with amounts of nitrogen application in the agricultural system. The rate of nitrogen loss in the vegetable production system can reach 52-75% (Li et al., 2003; Zhu et al., 2005) and the data of conventional crop systems depends on many practical factors. The nitrogen loss paths in two planting patterns are also quite different.
Leaching, ammonia volatilization and denitrification are the main nitrogen loss pathways for both paddy and vegetable production systems.
Paddy fields are more likely to have high ammonia volatilization loss induced by their flooded water. Many studies have reported that ammonia volatilization is the main nitrogen loss pathway in paddy fields (Cai, 1997; Wang et al., 2007; Xue et al., 2011; Xu et al., 2012),
12
accounting for up to 40% of the nitrogen input (Zhu and Chen, 2002). Additionally, it is also an important component of nitrogen loss in maize-wheat systems due to alkaline soil (Qian et al., 1997; Wang et al., 2004). A few studies have examined ammonia volatilization in vegetable production systems and found nitrogen loss from ammonia volatilization was relatively low (Liu et al., 2003; Xi et al., 2010; Min et al., 2011b). Soil planted with vegetables is usually more acidic than that planted with paddy. Ammonia volatilization could be remarkably restrained when the soil environment becomes acidified. The acidification trend may be used to explain the low rate of nitrogen loss via ammonia volatilization in vegetable production systems (Haruna Ahmed et al., 2008).
Due to large soil residual nitrogen and frequent irrigation, vegetable production systems have a greater potential for nitrogen leaching compared to conventional crop systems (Thompson et al., 2007). For example, data from China’s Yili river catchment revealed that vegetable fields in that region contribute 18.8% of the total aquatic nitrogen load to agricultural non-point pollution with only 8.7% of total planting area (Luo et al., 2015). Previous studies showed that leaching is the primary nitrogen loss pathway in vegetable production system, accounting for over 70% of the total nitrogen loss (Min et al., 2011a). Min et al. (2011b) planted three vegetables successively and found the nitrogen loss via leaching was 9-17 kg N/ha, 84- 232 kg N/ha and 16-32 kg N/ha in tomato, cucumber and celery growing seasons, respectively.
The considerable nitrogen loss via leaching ranging from 43-328 kg N/ha per year was also found in the maize-wheat system (Liu et al., 2003). Additionally, bases such as K+, Ca2+, Na+, Mg2+ get lost with nitrate nitrogen leaching, accelerating acidification in vegetable soil (Yin et al., 2004; Han et al., 2014).
However, nitrogen loss via leaching in the paddy-wheat system is quite low, estimated at 3-27 kg N/ha per year, accounting for 2% of nitrogen fertilizer application rates (Xing and Zhu, 2000; Zhu et al., 2000; Zhu and Chen, 2002).
Another possible pathway of nitrogen loss from vegetable production systems may be denitrification (He et al., 2007; Mei et al., 2009; Deng et al., 2012). More nitrogen loss may occur via denitrification from vegetable production systems than that lost from conventional
13
crop systems because of a high nitrogen fertilizer application rate, and proper aerobic conditions in vegetable production systems can provide major substrates for nitrous oxide production (Bouwman et al., 2002). Previous studies provide data that the loss rate of nitrous oxide emissions of the nitrogen input ranges from 0.3 to 4.9 % in rice-wheat systems (Zhu and Chen, 2002; Zou et al., 2005; Yao et al., 2009) and from 0.1 % to 11 % in vegetable production systems (Mei et al., 2009; Deng et al., 2012). Sometimes denitrification losses are negligible, and the variety of soil denitrification partly depends on changes in rainfall (Liu et al., 2003).