1.3 Nitrogen processes in the vegetable production system
1.3.1 Comparison of nitrogen input in a vegetable production system to that in a
Nitrogen mineralization, fertilizer application and deposition are regarded as nitrogen inputs supplying mineral nitrogen for plant growing.
1.3.1.1 Soil nitrogen mineralization and nitrification
Nitrogen in the soil is mostly organic nitrogen; only a small part of the soil nitrogen pool is mineral nitrogen. Almost all crops depend on mineral nitrogen supplementation; in other word, mineral nitrogen is the available form for crop uptake. Soil nitrogen mineralization is the most important nitrogen process, decomposing organic nitrogen into mineral forms that may be plant-available. Thus, soil nitrogen mineralization is regarded as the predominant nitrogen input process for the agricultural system.
Essentially how much mineral nitrogen the soil can support for crop growth is determined by the nitrogen mineralization rate during the growing period. It is difficult to compare the real nitrogen mineralization rates in situ between vegetable production systems and conventional crops systems. Soil from vegetable fields seems to have a higher mineralization rate than that from conventional crops fields because fertilizer utilization efficiency in vegetable production system is lower than that in conventional crops systems (Zhu et al., 2005; Min et al., 2011b).
As a result, nitrogen mineralization processes in a vegetable production system have to meet the vegetable nitrogen demand (Li et al., 2003).
Soil properties can have an effect on the nitrogen mineralization rate. Firstly, the ratio of soil total carbon to total nitrogen (TC/TN) has a negative correlation with nitrogen mineralization rate (Sun et al., 2002). Most soil from vegetable fields has lower TC/TN than soil from conventional crop fields due to its high nitrogen inputs, inducing higher soil nitrogen mineralization rates. Secondly, although soil nitrogen mineralization is regarded as being insensitive to acidity, enhancement of nitrogen mineralization is commonly reported after liming of acid soils (Jin et al., 2005a). Acidification is one of the common soil problems occurring in vegetable soil. It implies that soil nitrogen mineralization rates in vegetable
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production system may be reduced when soil becomes more acidic. Thirdly, soil nitrogen mineralization rate is also affected by soil moisture content. Soil nitrogen mineralization rate increases with soil moisture content increase till the 100% WFPS (Water-Filled Pore Space) in aerobic conditions (Jin et al., 2005a). However, anaerobic conditions will weaken and even restrain the soil nitrogen mineralization process. Negative nitrogen mineralization has even been found in paddy systems due to their flooded environment.
The nitrification process alternately regulates mineral nitrogen forms from ammonium to nitrate and nitrite (trace amounts). Indeed, a high nitrification rate has often been considered as an index of soil fertility, because most crops (except paddy) are inclined to use nitrate as an accessible nitrogen source from soil (Haynes, 2012). Both soil chemical and physical properties have significant effects on the nitrification process when there is enough ammonium existing as substrate. It has been observed that nitrification has a positive relationship with soil pH, and stops when soil pH is less than 4.5 (Fan and Zhu, 2002). The soil moisture content is favorable for nitrification at 50 % - 60 % of WFPS (Water-Filled Pore Space) (Pihlatie et al., 2004;
Bateman and Baggs, 2005).
1.3.1.2 Nitrogen fertilizer application
Fertilizer is the main source of nitrogen in the vegetable production system. Compared to the conventional crop system, 2-3 times greater amount of nitrogen fertilizers is applied as a nitrogen resource in order to ensure high yields in the vegetable production system in China and the amount of nitrogen application has been 4-8 times higher than that of plant demand (He et al., 2007). An investigation conducted in the north plains of China found that the average amount of nitrogen fertilizer is 1100-1500 kg N/ha in vegetable fields per year, whereas less than 500 kg N/ha was used in maize-wheat fields (Ma et al., 2000; Ju et al., 2009). Data from the Tailake region of China revealed that vegetable fields receive 900-1300 kg N /ha from nitrogen fertilizer, whereas paddy-wheat fields receive 400-600 kg N /ha (Huang et al., 2006;
Zhu et al., 2006; Min et al., 2011b). Furthermore, organic manures are often applied randomly and most farmers do not consider organic manure as part of nitrogen fertilizer in vegetable fields (Huang et al., 2006; Ju et al., 2006). Nitrogen input from fertilizer in the vegetable
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production system is 3-4 times higher than that in conventional crop systems (He et al., 2007).
Despite the rate of nitrogen fertilizer application being varied due to different vegetable types and multiple cropping indices, excessive nitrogen fertilizer application is very common in intensive vegetable production systems in China. The amount of nitrogen fertilizers applied is usually much higher than the recommended amount in vegetable production systems (Zhu et al., 2006).
When excessive nitrogen is applied in the vegetable production system, hydrogen ions, produced from the nitrogen nitrification process, gather in the soil profile and thereby increase soil acidity. The decline (0.7-1.96 unit) of soil pH values has been reported after the soil used to plant conventional crops was converted for planting vegetables (Yin et al., 2004; Zhang et al., 2005; Sun et al., 2006). If farmers are not given scientific advice they may apply more nitrogen fertilizer to maintain the yields in the following year, making soil acidity worse.
1.3.1.3 Other input
Irrigation water, wet deposition and seeds are also nitrogen inputs for the agricultural system. Nitrogen from wet deposition and seeds are very little in most condition of both vegetable production system and conventional crops system. However, contributions of irrigation water to nitrogen input are dependent. Compared to crops planted in dry fields, vegetables need a considerable amount of water for growing. Irrigation times and amounts of irrigated water used are more than for conventional crops planted in dry fields. An investigation conducted in the China north plain found that nitrogen from irrigation water input to the vegetable production system is 84 times higher than the input to the maize-wheat system; the nitrogen contribution from irrigation water represents over 10% of the nitrogen input in the vegetable production system, whereas the nitrogen contribution from irrigation water is less than 1% of the total nitrogen input in maize-wheat system (Ju et al., 2006). However, when compared to paddy planting in flooded conditions, results are very different. Studies in China’s Tailake region found that the nitrogen contributions of irrigation water are less than 1% and 3- 6% of the nitrogen input in the vegetable production system and paddy-wheat system, respectively (Min et al., 2011b; Xue et al., 2011). Not only the volume but also the nitrogen
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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