Nitrogen and root zone temperature effects on sweetpotato cultivation on large mounds

Field: Agricultural and Biological Sciences (Life Sciences)

Sweetpotato (Ipomoea batatas) is an important food crop grown widely throughout the world. In Papua New Guinea (PNG), sweetpotato is a staple food, with more than two-thirds of total production occurring in the highlands region. Although mound cultivation is commonly used for sweetpotato production throughout PNG, a unique cultivation method is used in the Western highlands provinces whereby sweetpotato is grown on a large mound amended with plant residues. Decomposition of these residues releases nitrogen (N) and increases mound temperature, which is suspected to improve storage root production of sweetpotato. This culture is a near permanent system and has the potential to intensify production throughout the region. However, even though large mound culture is a well-documented cultivation system, little research has been conducted to investigate the exact mechanisms affecting sweetpotato yield. This research examines two factors, N nutrition and root zone temperature (RZT), that are thought to be main drivers for improving sweetpotato production under the large mound culture.To investigate N and RZT effects, three different studies were conducted. These were designed to examine the effect of (i) N supply on sweetpotato growth and yield in the glasshouse and the field; (ii) RZT on sweetpotato growth and yield under glasshouse conditions; and (iii) the combined effect of N supply and root zone temperature on sweetpotato growth and yield in the field.The first study investigated the effect of N supply on growth and yield of Beauregard sweetpotato cultivar in glasshouse and field trials. In the glasshouse trial, with staked plants, there was a linear increase in the number and dry-biomass of storage roots in response to increasing N supply up to a rate of 130 kg ha-1, while further increases of N above optimum rates (>130 kg ha-1) did not change any of these attributes. With increasing N supply, the dry-biomass of storage roots linearly increased with aboveground biomass and leaf area; indicating plants could maintain high photosynthetic capacity of the canopy. In the field trial, N supply had no effect on the number of storage roots formed, and increasing N by more than 260 kg ha-1 reduced the total yield of sweetpotato by up to 14%. This yield reduction was largely due to excess growth of the aboveground biomass, which increased respirational demand at the expense of storage root yields. In this study, while there was no evidence that high N supply inhibited storage root formation, high N supply significantly reduced storage root yield of sweetpotato in the field.The second study investigated how the formation and dry-biomass accumulation of storage roots was affected by i) a constant elevated RZT in Beauregard cultivar, and ii) a short-term increase in RZT in Beauregard and Northern Star cultivars. A split root system was employed for plant cultivation, whereby a growing pot was divided into a control (RZT kept constant at 20°C) and treated compartment (RZT increased to 30, 40, or 50°C). The results showed that a constant elevated RZT did not affect the total number of storage roots formed in Beauregard, but a constant RZT of 30°C and above reduced the total storage root dry-biomass production. The short-term increase in RZT had no effect on the storage root number or biomass of Beauregard, but did reduced storage root formation and dry-biomass production in the Northern Star at a RZT of 50°C. These results suggest that an elevated RZT inhibits dry-biomass accumulation in storage roots, with a more pronounced effect occurring at a constant elevated RZT. This indicates that the crude manipulation of RZT that can be achieved through the breakdown of plant residues amended to the large mounds would be as likely to adversely affect yield, as it would be to increase yield.To further examine the mechanism responsible for increasing sweetpotato yields in the large mounds, a third study was carried out to investigate the influence of the type of land preparation (mounds versus flatbeds) and soil fertility management (lucerne (Medicago sativa) addition versus fertiliser addition) on the growth and yield of Beauregard and Northern Star sweetpotato cultivars. The final yield of sweetpotato grown with fertiliser addition was 20% greater than that in plants grown with lucerne addition. This was associated with an up to 31% greater absolute growth rate (AGR) and 24% greater relative growth rate for plants grown using fertiliser. than plants grown using lucerne addition at 4 to 8 weeks after planting (WAP). Although neither land preparation nor soil fertility management had any effect on leaf area index (LAI), there was a significant interaction between land preparation and soil fertility with sweetpotato grown on the mounds with lucerne addition having a significantly lower LAI than other treatment combinations at 8 WAP. The RZT of the mounds amended with lucerne also tended to be up to 8oC higher than that observed in the other treatments until 8 WAP. Land preparation and cultivars had no effect on storage root yield. However, an approximately twofold greater AGR, LAI and N accumulation resulted in greater aboveground and total plant dry-biomass in the Northern Star compared to the Beauregard cultivars. It is believed that a mismatch between N supply and plant growth occurred in lucerne addition treatments, and that this negatively affected photoassimilate production and allocation to storage roots. In addition, the high RZT could have changed the metabolism of photoassimilates by enhancing storage root respiration, leading to reduced dry-biomass accumulation of the storage roots.Overall the results from these studies indicated that large mound culture may not be a cultivation system that will consistently improve sweetpotato production. This could be largely attributed to the mismatch between N supply and plant demand, and the negative impacts of elevated RTZ on storage root growth during the early growing season. Synchronizing the sensitive stage of plants with favourable conditions during the growing season would help to maintain or improve sweetpotato yield production on the large mound culture.