Estimating the 1974-2015 Trend of Carbon Input By Crops into Agricultural Soils for Whole of Canada.

Quantifying the temporal changes in large-scale agricultural carbon (C) budget is essential to understand greenhouse gas emission and mitigation. To estimate the C input from 1974 to 2015 for the 56.3 M ha of agricultural soils in Canada, we developed a method to estimate changes of C input by plants into Canadian agricultural based on harvested product. A relationship between harvest index (HI) and grain yield for 11 major crops was established by reviewing field studies conducted in Canada with information about HI and grain yield (n = 415). We found significant linear relations between HI and grain yield for wheat, maize, barley, pea, chick pea, lentil, soybean, canola, and flax (R² = 0.327–0.651) with changing rates of HI ranged from 0.015 for maize to 0.102 for flax by 1 Mg ha⁻¹ yield increase. Furthermore, an accurate accounting of root and rhizodeposition C input is required due to the incomplete and inconstancy of root sampling depth and corresponding root to shoot (R/S) ratio estimate. A dataset was compiled to make adjustment of R/S ratio with soil depth for major crops and forages in Canada (n = 211) by using root distribution patterns for different plants. The adjusted mean R/S ratios for crops in the whole profile were highest for oat (0.419), followed by canola (0.375), and lowest for potato (0.097). Forages showed much higher R/S ratios than grain crops, in which alfalfa (and alfalfa mixtures) had higher R/S ratio (1.049) than other forages (0.737). Generally, C input has increased over last 40 years due to changes in species of plants grown on agricultural land and changes in yields. The amount of change varies across the country because these changes are uneven.