nutrient cycling

Question 1

desribe the N cycle

Answer 1

1. largest pool in atmosphere, N2 taken up by biological N2 fixation (also atmospheric deposition of nitrate and ammonium and NOx through lightning)
2. N2 that is going through fixation is then turned into ammonium (NH4) with the help of bacteria and enzyme called nitrogenase
3. eventually, that N that was fixed will decompose and turn into dissolved organic nitrogen (DON) which is either taken up by plants or mineralized to become NH4 -> becomes immobilized, adsorped into the soil matrix, occluded, volatilized or undergoes nitrification
4. nitrification: with help of nitrosomonas (ammoniumoxidizer), NH4 becomes nitrite (nitrous oxide emissions) and then nitrite -> nitrate with nitrobacter (nitriteoxidizer)
5. from there, there is either immobilization of nitrate, leaching of nitrate, accumulation of nitrite because too cold or pH too high for nitrateoxidase OR denitrification
6. denitrification is nitrate -> nitritite -> nitrogen monoxide -> nitrous oxide -> molecular nitrogen (emissions of nitrogen monoxide, nitrous oxide and molecular nitrogen) under anaerobic conditions

Question 2

describe the P cycle

Answer 2

• soil mineral P: solubilized to become bioavailable phosphate (calcium phosphates - apatite- more soluble with lower pH and Fe and Al phosphates more soluble with higher pH) which is then leached or uptook by plants or microorganisms (most available for fixation at pH 6-7). That phosphate can then also be sorped back into soil mineral P
• soil organic P: comes from crop residues (which are sometimes eroded away) mineralized to become bioavailable phosphate or leached. that phosphate can then be immobilized and turned back into soil organic P.

Question 3

describe the distribution of soil phosphorus during soil development

Answer 3

1. early stages: all P from primary minerals
2. bioavailable and organic P increase
3. very developed soils: occluded P increases because P is taken up by Al and Fe oxides due to loss of base cations
4. terminal steady state: soil only consists of organic P and occluded P
   - organic P is mineralized (taken up immediately by plants due to high competition)
   - atmospheric depositon of dusts balances these losts

Question 4

what are the functions of calcium in the soil?

Answer 4

• macronutrients for plants
• improves availability of P because more base cations so Fe and Al do not need to cling on to P
• increases rate of mineralization of OM
• improves soil structure by reacting with organic acid
• high leaching potential