Residual Effect of Green Manure on Soil Properties in Green Manure-Transplant Aman-Mustard Cropping Pattern

The intensive cultivation as well as monoculture of rice has brought a positive impact on food production in Bangladesh, nevertheless it has resulted in a serious depletion of soil fertility. In Bangladesh, most soils have less than 1.5% organic matter. However, excessive use of chemical fertilizers has led to several issues such as serious soil degradation, nitrogen leaching, soil compaction, reduction in soil organic matter and loss of soil carbon. Lin et al., (2019) found from a study and stated that the contents of heavy metals (Cu, Pb, Cd, As) were lower in soil and plant samples under organic fertilizer treatment compared to those treated with chemical fertilizer. Intensive land uses with continuous growing of similar crops significantly affect soil health and crop growth. In this context, green manures based cropping pattern is the way of hope as an alternative of inorganic fertilizer. Green manure helps in gaining back the deteriorated soil quality.  The effect of decomposition of a green manure crop can be largely attributed to the increase of plant nutrients availability in soil and to the following crop. Sesbania green manures could have benefits for soil N dynamics by recovering residual mineral N in soil, by fixing N from the atmosphere for leguminous green manures and thereby contributing to subsequent crop N nutrition (Becker et al. 1995, Griffin et al. 2000). Similarly, Cavigelli and Thien (2003) postulated that incorporating green manure crops into soil might increase P bioavailability for succeeding crops. Incorporation of Sesbania rostrata and Sesbania aculeata added more organic matter and nitrogen to the soil than the prior soil (Irin et al., 2019) and reduced 50 percent of recommended N-levels of rice (Irin et al., 2020). Application of green manure plus chemical fertilizers is found to produce significantly higher yield than that of sole application of chemical fertilizer (Aktar et al., 1993). Ali et al. (2012) stated that the green manuring and leguminous cropping patterns gave higher paddy yield compared to rice- wheat cropping pattern. To reduce production cost and improve soil and crop productivity, integration of legume cover crops in cropping systems is now being highly emphasized among farmers in the tropics (Odhiambo et al., 2010). In Bangladesh, a few research works was done scattered but intensive work should be undertaken to study the different green manure crops based cropping pattern and its residual effect on subsequent (T. aman) and following soil (Mustard). Considering the above facts, the present experiment was undertaken to study the residual effect of different green manuring crops on soil properties in green manure- T. aman- Mustard cropping pattern.
 

The experiment was carried out at Sher-e-Bangla Agricultural University field during 2015 to 2016. The soil of the experimental field was sandy loam in texture having pH 5.9. 
 
Cultivation of green manure crops during the fallow period prior T. aman rice and mustard cropping season
 
During 1st week of May, the eight green manure crops viz. Deshi dhaincha (Sesbania aculeata), African dhaincha (Sesbania rostrata), Sunnhemp (Crotolaria juncea), mungbean (Vigna radiata), blackgram (Vigna mungo), cowpea (Vigna unguiculata), Ipil-ipil (Leucaen aleucocephala) and Mimosa (Mimosa pudica) sown from seed and in situ incorporated at 45 DAS. The final soil samples of each experimental plot (0-15 cm) were collected and analyzed at 30 days after decomposition. The experiment was laid out in a randomized complete block design with three replications.  The size of each plot was 17.50 m2 (5 m x 3.5 m).
 
Experimental design and layout for rice and mustard
 
Seedling of (35 days old) T. aman rice (BRRI dhan 66) were transplanted on 13 July and BARI Sarisha-14 were sown on 25 November. The recommended fertilizer used for T. aman and mustard were 200-74-100-67-10 kg and 83-30-57-21-2 kg N, P, K, S and Zn ha-1 from their sources of urea, triple super phosphate, murate of potash, gypsum and zinc sulphate respectively. To supply nitrogen, two levels of urea i.e. 100% urea and 50% Urea from their recommended doses were applied in three equal splits. Two fertilizer doses and eight types of green manures were tested on rice (succeeding crop) and mustard (following crop) in a split-plot design with three replications.
 
Soil analysis
 
After harvesting T. aman and mustard crops, final soil sample were taken following standard methods. Soil organic matter was determined by wet oxidation method total N by modified Kjeldahl method, available phosphorus by the Olsen method.
 
Data analysis
 
The collected data were analyzed statistically by using the Statistic-10 computer package. The mean comparisons of all parameters were done with Tukey’s W-procedure (Gomez and Gomez, 1984).

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
 
Incorporation of eight different green manures increased soil organic matter from 1.01% (initial) to 1.08% in 1st year and 1.61% in 2^nd 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 1^st 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
 
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
 
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
 
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%).

Soil potassium
 
Post-harvest rice (both year) and mustard (1^st 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). 
 
Soil phosphorous
 
In 1^st 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 2^nd 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 (H₂CO₃) 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).

The Residual effect of green manure improved soil fertility in GM- T. aman- Mustard cropping pattern by adding organic matter, nitrogen, P and K to the soil resulting reduced fertilizer cost. For betterment of soil, GM - T. aman - Mustard cropping pattern could be practiced instead of Fallow - T. aman - Mustard cropping pattern. The 50% reduction of N   and P fertilizer can be recommended for succeeding T. aman rice from followed by growing Sesbania rostrata, Sesbania aculeata, Crotalaria juncea or Vigna unguiculata and L. leucocephala as a preceeding green manuring crop. As recommended cropping patterns needed 207 days, so it can be possible to include another short duration crop like mungbean in existing cropping patterns during the gap between mustard and green manuring crops. Hence in such case, GM- T. aman- mustard-mungbean may be a more effective cropping pattern in respect of yield and soil quality concern that can be tested in future studies.

The financial support of the authority of Syed Momena Montaj Foundation during research work is gratefully acknowledged.