2.1 Site description and fertilization strategies
Soil samples were collected in April 2015 (before early rice
transplanting) from long-term fertilization experiment stations at
Ningxiang in the subtropical region of the Hunan Province, China
(111°54’−112°18’E, 28°07’−28°37’N). The mean annual temperature and
rainfall of the site are 17°Cand1681 mm, respectively. The soil was
derived from river alluvium, Quaternary red clay, and plate shale, and
the cropping system used was milk vetch–rice–rice. The main physical
and chemical properties of the soil at the start of the long-term field
fertilization trials were as follows: pH 5.8, SOC17.1 g
kg−1, total N 1.76 g kg−1, available
N 144mg kg−1, and Olsen-P 12.8 mg
kg−1 (Yuan et al., 2012).
The experiment started in 1986 with the following four treatments: no
fertilizer (CK), mineral fertilizer (NPK) (urea, superphosphate, and
potassium chloride), rice straw combined with mineral fertilizer (NPK +
ST), and 70% NPK +30% chicken manure (NPK +OM). The rice straw
produced from the straw plots was returned to the corresponding plots in
July and November after the early and late rice harvests at rates of
2775 kg ha−1 a−1 and 3600 kg
ha−1 a−1, respectively. The
experiment design ensured that all fertilizer treatments received the
same amount of N, phosphorus pentoxide
(P2O5), and potassium oxide
(K2O). The total amount of N,
P2O5, and K2O was the
sum of the chemical fertilizer plus that from the rice straw residue or
manure, corresponding to143 kg N ha-1, 23.2 kg
Pha-1, and 52.3 kg K ha-1; and 158.0
kg N ha-1, 18.6 kg P ha-1, and 67.2
kg K ha-1per fertilized treatment for the early and
late rice seasons every year, respectively. The total amounts of
fertilizer applied in each fertilized treatment during the milk vetch
season were identical to those for the late rice season.