Soil organic carbon and nitrogen stocks in Nepal long-term soil fertility experiments

Loss of organic C from tilled agricultural soils has generated interest in C-sequestration in soils. The effects of nutrient and organic inputs on soil organic carbon (SOC) and total nitrogen (TN) stocks were studied in three 23-25-year-old long-term conventional tillage fertility experiments (LTFEs) in the Nepal terai. The sites differed in soil texture and were cropped annually to rice (Oryza sativa L.) and wheat (Triticum aestivum L.). Treatments were control (unfertilized), recommended NPK, and farmyard manure (FYM; 4 Mg dry wt./crop) at all sites, plus FYM (4 Mg ha―1 year―1) or chopped wheat straw (8 Mg dry wt. ha―1 year―1) plus N to rice and NPK to wheat at Parwanipur. Small areas of unfertilized grassland created at the same time as the LTFEs were sampled to provide a no-tillage comparison. Soil samples were collected at 15 cm intervals to a depth of 60 cm. In general, SOC and TN stocks were highest in the FYM treatment and the grassland, intermediate in treatments with residue return or reduced FYM inputs and lowest in the control and NPK treatments. Differences in SOC and TN stocks were found in the top 30 cm of soil at Bhairahawa but only in the top 15 cm at the other two sites. Application of inorganic nutrients increased SOC and TN only at Bhairahawa, which had the highest silt + clay content and where severe P deficiency was observed without fertilization. SOC stocks to 60 cm at Bhairahawa were 33.5, 36.8 and 53.1 Mg C ha―1 for the control, NPK, and FYM treatments, respectively, on an equal soil mass basis. The SOC stock in the NT-grassland was 51.8 Mg ha―1, similar to that in the FYM treatment. Stocks of SOC and TN were anomalously high at Tarahara but differences between treatments were similar to those at Parwanipur, which had a similar soil texture. Retention of added carbon over the duration of the experiment was about 10% for FYM and 4.5% for wheat straw. It was estimated that 47% of the original soil C under native forest had been lost from the NPK treatments at Bhairahwa and Parwanipur and that recovery was to 71―76% of the forest value with addition of FYM. However, the rates of FYM used would not be achievable at a landscape scale. Given these results, shifting to no-tillage agriculture is probably the most practical option for increasing SOC and TN stocks.