Study site
The study was conducted at the farmer’s field in Jomsri village, Kalasin province and Muang village, Khon Kaen province of Northeastern Thailand from January 2018 to November 2019 by faculty of Agriculture, Khon Kaen University. The climate of the Northeastern Thailand has a semi-humid tropical climate, which characterized by rainy (May-October) and Dry (November-April) season
(Goto et al., 2008). The monthly rainfall, average temperature, relative humidity and evaporation during cropping season of two study sites are shown in Table 1.
Soil characteristics
Soil samples were collected from the areas and analysis for chemical and physical properties (Table 2). The surface soil was strongly acidic, low organic matter, total N, available P and exchangeable K of both the study sites. The soil samples were classifying as sandy loam and sandy in Jomsri and Muang village sites, respectively.
Experimental design and treatments
The paddy field is plowed and incorporating the remaining rice straw to the soil. The treatments were set up in a randomized completely block design with four replications. Details of treatment combination are presented in Table 3.
Groundnut management
Groundnut variety Tainan 9 was grown in order to provide economic return and residues in treatments 3 and 4 in dry season (January-April). The crop grown under limited irrigation from harvested rainwater ponds and carry-over residual moisture after the harvest of rice. Two times irrigations were provided at peg initiation and grain filling stages. Lime (gypsum) was applied at rate of 312 kg ha
-1 by incorporation to the soil during the last plowing. Application of gypsum to peanut increased seedling vigor and seed yield
(Annadurai et al., 2009). The crop was sown on the second week of January when a sufficient amount of carry-over residual moisture was available in the fields. Chemical fertilizer grade 15-15-15 (N-P
2O
5-K
2O) was applied as side dressing to the crop at rate of 156 kg ha
-1 at 10 days after seeding. Manual weeding was done twice before pegging initiation stage. The crop was harvested at 101 days after seeding. Groundnut pods were removed from the stem and then the stover was incorporated to the soil in treatment 3 and 4.
Groundnut stover biomass and nutrient composition
At harvest time, groundnut stover were sampled to determine the dry weight per unit of land area and analysis for total N (%), total P (%) and total K (%). The nutrients returning to the soil calculated by stover dry weight (kg ha
-1) × nutrient content (%) and divided by 100. The stover dry weight obtained 4.89 and 2.24 ton ha
-1 in Jomsri and Muang village site, respectively. The total N, total P and total K contents of the groundnut stover were observed 0.674%, 0.057% and 1.591%, respectively in Jomsri. While, the total N, total P and total K contents of the groundnut stover were attained 0.434%, 0.028% and 2.405%, respectively in Muang village site.
Weed biomass and nutrient composition
Weeds were randomly sampled one square meter from fallow unweeded treatment plots to determine aboveground dry weight and nutrient content of total N (%), total P(%) and total K (%). The nutrients returning to the soil calculated by aboveground dry weight (kg ha
-1) × nutrient content (%) and divided by 100. In the present experiment, weeds dry weight obtained 1,612.5 and 475.6 kg ha
-1 in Jomsri and Muang village sites, respectively. The N, P and K content of weeds are 1.047, 0.209% and 1.870%, respectively in Jomsri. While, the N, P and K content of weeds are 1.083%, 0.090% and 1.995%, respectively in Muang village site.
Rice succeeding groundnut management
After-plowing groundnut stover (T3 and T4) and weed (T1 and T2) residues to the soil for 30 days, the paddy fields were plowed again suitable to rice transplanting. Paddy bunds were constructed surrounding areas of 4 × 6 m treatment plots. Rice variety KDML105 seedlings (30 days old) were transplanted at a spacing of 25 × 25 cm, 4 plants hill
-1. Hand weeding was done twice at 30 and 60 days after transplanting. For the treatment 2 (farmer’ s management), fertilizer grade 16-16-8 was applied at rate of 219 kg ha
-1 by split 2 times at 30 days after transplanting (156 kg ha
-1) and at panicle initiation stage (63 kg ha
-1). For the treatment 4 incorporation groundnut stover (improved management), fertilizer grade 16-16-8 (N-P
2O
5-K
2O) was applied at rate of 109 kg ha
-1 (50% of farmer practice) by split 2 times at 30 days after transplanting (77 kg ha
-1) and at panicle initiation stage (32 kg ha
-1). Rice was harvested at 130 days after transplanting in both study sites.
Growth and yield of succeeding rice
The growth characteristics such as tiller number, leaf area and aboveground dry weight were measured at panicle initiation stage. Yield components such as panicle number, filled grain number and 1000 grain weight were determined at harvest. Grain yield was measured from harvesting area of 3x4 meter and express as kg ha
-1 at 14% moisture content.
Statistical analysis
All data of rice growth, yield and yield components were subjected to analysis of variance (ANOVA) at first, then the significance of differences between the treatments was determined by least significant difference (LSD)using Statistix 10 software (Analytical software, 2013).