Soil chemical properties
The chemical properties of initial soil are shown in Table 1. The pH, organic matter, nitrogen, P and K concentration of initial soil were taken before green manure sown.
Effect of in situ green manure incorporation on soil properties at two consecutive years
Table 1: Initial soil data prior cropping.
Incorporation of eight different green manures increased soil organic matter from 1.01% (initial) to 1.08% in 1st year and 1.61% in 2nd
year (2016). The 0.14% higher organic matter was found in T2 compared to T7 in 2015 and 0.5%, 0.4% and 0.3% was recorded from T1, T2, T5 in 2016 (Fig 1). Total N status of soil increased from 0.04% (initial soil) to 0.084% in 2015 and 0.081% in 2016 (Fig 1) and T2, T6 showed the increased trend followed by T5 and T7 in both year. K showed slightly increasing trends (0.22 meq/100 g) from initial soil (0.18 meq/100 g) in 1st
year but it decreased in 2nd year (Fig 1). The nutrient balance of soil after incorporation of different green manuring crops specially S. rostrata, S. aculeata
and C. juncea
showed positive balance of nutrients than other green manures (Irin et al., 2019).
Effect of green manuring crops and nitrogen levels on grain weight, protein content of rice and yield of rice and mustard
Fig 1: Effect of different in situ green manuring on soil organic matter (%), N (%), P (ppm) and K (meq/100 gm) contribution on pre sown rice soil in 2015 and 2016. Here, T0 = Control, T1 = S. aculeata, T2 = S. rostrata, T3 = C. juncea, T4 = V. radiata, T5 =V. mungo, T6 =V. unguiculata, T7 = L. leucocephala, T8 = M. pudica.
The combined effect of NPK levels and residual effect of green manuring crops had a significant influence on 1000 grain weight, protein content and grain yield of rice and mustard (Table 2). In both years, the highest 1000-grain weight of rice was obtained from the treatment combination of T2 with F1 which was statistically similar to F2 levels. The highest T. aman yield, mustard yield and grain protein (5.56 t ha-1
with F1 and 5.11 t/ha with F2), (1592.3 kg/ha) and (8.54%) was obtained from T2 followed by T1 and T3 which was statistically similar to the combination of F2 fertilizer in 2015 and 2016. The nutrients contributed from legumes could be partly responsible for yield improvement and up to reduction of 50% inorganic N fertilizer. The increased grain yield and protein content may be due to more availability of nitrogen and other nutrients to rice crop released by incorporation of green manure. This result supported by Ehsan et al. (2014)
who stated that, the rice grain yield increased 32% to 77% over control due to green manure (dhaincha) incorporation with different doses of NPK fertilizers application.
Residual effect of green manure and N fertilizer on post-harvest rice (succeeding crop) and mustard (following crop) soil
Soil organic matter
Table 2: Interaction effect of fertilizer levels and different green manuriing crops on grain yield, 1000-grain weight, protein content of transplant aman rice yield in two years.
After two years (post-harvest rice soil) SOM was increased 0.8% and 0.3% (T1) and 1% and 0.6% (T2) with F1 and F2 whereas 1.21% (T1 and T2) increased in post-harvest mustard soil followed by T3 (1.08) with F1 (Fig 2). Soft part of green manure crops are easy to break down wherever more recalcitrant molecules like lignin takes a longer time of decomposition and thus nutrients reserve in soil and release latter (Berg et al., 2007)
and thus continuous cultivation of GM crops increased soil OM varied from 2.30% to 2.95% at initial stage and 2.71% to 2.98% at postharvest stage (Chanda and Sarwar, 2017)
. The incorporation of Sesbania
improved the soil nutrient status and residual effects lasted up to 3 years (Onim et al., 1990).
Total soil nitrogen
Fig 2: Residual effect of green manuring crops on (A) organic matter (%), (B) soil N (%), (C) K (meq/100 g soil) contribution in post- harvest rice and mustard soil in 2015 and 2016. Here, T0 = Control, T1 = S. aculeata, T2 = S. rostrata, T3 = C. juncea, T4 = V. radiata, T5 =V. mungo, T6 =V. unguiculata, T7 = L. leucocephala, T8 = M. pudica. Here, F1= Recommended dose for N in 2015 and NPK in 2016, F2= Half of recommended dose for N in 2015 and NPK in 2016.
In post-harvest rice soil ( after two years), soil N increased 0.04% and 0.05% (F1 and F2) from T1 and T2 and 0.041 to 0.094% in post-harvest mustard soil (Fig 2). It may be attributed to the mineralization of N by organic manure in soil and greater multiplication of soil microbes, which could convert organically bound N to inorganic form. Rahman et al. (2013)
showed the similar results and stated that total N status of soil ranged from 0.07 to 0.09% (initial level 0.07%).
Post-harvest rice (both year) and mustard (1st
year) soil shown little decreased trend of soil K from initial soil (0.18 meq100 g-1
) but drastically increased trend was observed in 2nd year from T1,T2, T6 and T8 showed the highest result. (Fig 2). The higher exchangeable K obtained with green manuring along with K fertilizer was due to the extensive root systems of green manure crops may improve the physical condition of soil and liberated CO2 and organic acid which helped in dissolving native K in soil and thus increasing the availability of K (Singh et al., 2009).
year post-harvest rice and mustard soil shown decreased trend of soil P than the initial level (15.83 ppm) (Fig 3,4) whereas incredibly increased trend was observed in 2nd
year from T1 (7.8 ppm in F1 and 11.31 in F2 ppm), T2 (16 ppm in F1 and 11.14 in F2 ppm) followed byT3. Again, T3 (31.30 ppm) followed by T7 (26.20 ppm), T2 (20.90 ppm) with F1 dose showed increased trend of P in post-harvest mustard soil. Green manure crops accumulate large amount of P and upon decomposition form bicarbonates (H2
) resulting solublize soil mineral P and makes the phosphorus sufficiency for the succeeding crops (Tissen et al., 1994).
Addition of green manures increases the soil organic carbon subsequently reduce soil pH which reduce phosphate fixation in soil with iron and aluminium and ultimately P availability increased (Dey and Nath, 2015)
. The excess soil P would help farmers to reduce fertilizer cost. Repeated application of green manure crops also result in decreased soil bulk density and increased soil aggregation and moisture retention, all factors that may help increase P uptake by succeeding crops via their effects on increased root and mycorrhizal growth (MacRae and Mehuys, 1985)
Fig 3: Residual effect of green manuring crops on soil P (ppm) contribution in post- harvest rice and mustard soil in 2015 and 2016. Here, T0 = Control, T1 = S. aculeata, T2 = S. rostrata, T3 = C. juncea, T4 = V. radiata, T5 =V. mungo, T6 =V. unguiculata, T7 = L. leucocephala, T8 = M. pudica. Here, F1= Recommended dose for N in 2015 and NPK in 2016, F2= Half of recommended dose for N in 2015 and NPK in 2016.
Fig 4: Interaction effect of pervious fertilizer levels and green manuring crops on seed yield of mustard in two pooled years. Here, T0 = Control, T1= S. aculeata, T2= S. rostrata, T3= C. juncea, T4= V. radiata, T5= V. mungo, T6= V. unguiculata, T7= L. leucocephala, T8= M. pudica.