4.2 Effect of N addition on soil characteristics
With the development of modern agriculture, nutrient addition
(especially N addition) has become an essential cultivation measure to
improve the productivity of most food crops and forages. However, the
influences of long-term fertilization on cultivated soil have also
aroused widespread concern in recent decades. For example, long-term
application of N fertilizer resulted in the decline of SOM in black soil
in Northeast China (Qiao et al. , 2007), and the aggravation of
soil acidification in Southern China, soil pH dropped by 0.5 units in
past 20 years (Guo et al. , 2010). So far, little research had
been done about the effect of N addition on the characteristics of
saline-sodic soil. Our results in saline-sodic paddy field showed that
the N addition level had no significant effects on soil pH and EC in the
short term (3 years), but the soil N contents were significantly
increased, soil pH and EC also decreased significantly with cultivation
time increasing (Huang et al. , 2016). In this study, soil pH, EC
and ESP decreased significantly when N addition rate was more than
ha-1 after 10 years of continuous N addition (Fig.4).
Similar results were also found in alfalfa pastures in North America and
mountain heathlands in Spain (Chen et al. , 2001; Marcos et
al ., 2003). These results further indicated that long-term continuous N
addition had a positive effect on reducing grassland salinization.
Many literatures have demonstrated that N addition improved soil
fertility, especially in some marginal lands (Wang & An, 2010; Liet al. , 2017; Kumar et al. , 2019). Our previous study also
showed that soil inorganic N and TN contents increased significantly in
the 0-20 cm soil layer of saline-sodic paddy field with the increasing
of cultivation years or N application rates (Huang et al. , 2016).
The experimental results indicated that the contents of SOM, TA and AN
under different N addition rates were significantly increased
(P <0.05) than those of no N treatment after 10 years of
continuous N addition in L. chinensis grassland (Fig.5).
However,compared with the experimental results of 7 years ago (Huanget al. , 2015), the contents of TA and AN in soils from different
N treatments had little change. Under the condition of continuous N
addition for many years in saline-sodic grassland, in addition to
increasing the hay yield of L. chinensis , where did the rest of
the N go? Why can’t soil N content be further improved? These problems
should be further studied in the future.
Interestingly, with N addition rates increasing, the content of SOM
increased significantly, while C/N ratio decreased significantly in
saline-sodic grassland of L. chinensis (P <0.05,
Fig.5A & 5D). The soil C/N ratio was maintained at about 15:1 when N
addition rate was more than 90 kg N ha-1, which was
close to the conventional value of SOM. Some studies had pointed out
that soil C/N ratio usually directly influences the nitrification and
denitrification process of soil microorganisms, and then affects the
conversion and discharge of soil N (Rizhiya, et al. , 2011; Kraft,et al. , 2014). In this experiment, soil C/N ratios were higher in
the original saline-sodic grassland without N addition and the treatment
with N addition rate less than 60 kg N ha-1, so the
growth of L. chinensis shoot was inhibited. Therefore, N addition
can increase the aboveground yield of plants and improve the quality of
plants/soils (Mou, 2015; Chang et al. , 2017).
Based on the analysis of soil enzyme activities related to the
transformation of soil C, N and P, it was found that the activities of
soil sucrase and urease increased significantly with N addition rates
increasing (P <0.05), while the activities of catalase
decreased, and there was no significant difference among different N
treatments (Table.1). Many previous studies have also demonstrated that
urease activity increased significantly and promoted N fertilizer
hydrolysis and mineralization under exogenous nN addition (Ge et
al ., 2010), thus, more absorbable N was provided for plants (Aerts &
Chapin, 1999). The similar changes of soil sucrase activity indicated
that exogenous N promoted plant growth and increased soil organic carbon
pool (Zhao et al. , 2008), while the response of soil catalase
activity to exogenous N addition was almost insignificant (Song et
al. , 2009; Wang et al. , 2015; Peng et al. , 2017).