Considering the dynamic changes of soil available nutrient
content, we found that AN significantly increased with time from
1998 to 2009 in OM and NPK treatments, suggesting that the
long-term soil organic matter played a major role in releasing soil
AN. The evident increase of AK in NPK treatment over time in
our study, suggests that continuously applying K fertilizer would
dramatically improve the soil AK supply, In addition, AP values in
both OM and NPK treatments were increased greatly compared
to the other three treatments during the whole experiment period.
This is consistent with many previous studies showing the
accumulation of P is the most obvious and long-term manure or